modify python SDK and update README

README.md

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-#gateway-python-sdk
+# gateway-python-sdk
 
-Python SDK for DevIoT gateway service
+### Python SDK for DevIoT service ([https://deviot.cisco.com/](https://deviot.cisco.com/))
+
+You can use this SDK to register devices to DevIoT and sync up data and actions between the devices and DevIoT.
+
+## Table of contents
+
+* [Getting Started](#getting-started)
+* [API](#api)
+
+## Requirement
+1. [Python](https://www.python.org/downloads/): This SDK is written in Python. We support both Python 2 and 3.
+2. [paho-mqtt](https://eclipse.org/paho/clients/python/): This SDK uses this library to build an MQTT client
+
+## Usage
+1. You can use SDK to register your own sensors and systems to DevIoT.
+2. Use [Starter-kit](https://wwwin-github.cisco.com/DevIoT/gateway-python-starter-kit) to run sample codes
+
+## Term
+
+- **Gateway**: 'Gateway' is a device connected to DevIoT like a Raspberry Pi board or a mobile phone
+- **Thing**: 'Thing' is a sensor or a module in a gateway like a light sensor, LED, or GPS. A thing is an icon in DevIoT. A gateway can have several things.
+- **Property**: 'Property' is a variable measured by a thing. For instance, Latitude and longitude are properties of a GPS. A thing can have several properties.
+- **Action**: 'Action' is a command for a module. For example, turning on and off LED is two different actions. A thing can have several actions.
+
+## Getting Started
+#### 0) Check the version of python (terminal)
+Before using this SDK, you need to check the version of python. The master branch is written in **Python 2**. You can check it with the following command.
+```
+python --version
+```
+
+If your python version is 3.x, you need to switch to 'python3' branch in the next step.
+
+
+#### 1) Set up SDK package (terminal)
+Clone SDK git repository
+```
+git clone https://github.com/ailuropoda0/gateway-python-sdk.git
+cd gateway-python-sdk
+```
+(**ONLY for Python 3 users**) Switch to Python 3 
+```
+git fetch origin python3:python3
+git checkout python3
+```
+
+Install SDK package on python
+```
+python setup.py install
+```
+
+#### 2) Connect gateway (python code)
+You can refer the example code in [gateway-python-starter-kit](https://wwwin-github.cisco.com/DevIoT/gateway-python-starter-kit).
+
+Import SDK
+```
+from cisco_deviot.gateway import Gateway
+from cisco_deviot.thing import Thing, Property, PropertyType
+```
+Construct a Gateway object
+```
+account = "your_id@cisco.com" # enter your DevIoT account
+app = Gateway(name="gateway_name", account=account)
+```
+
+Construct a Thing instance
+```
+thing = Thing("thing-id", "thing-name", "thing-kind")
+```
+A Thing instance can have both *properties* and *actions*. But you can only use it as *either* an input component *or* an output component in each project.
+
+In DevIoT project page, click the 'Connect' button and connect the thing component with a rules engine by double-clicking them. The type of the component is determined by which component is first double-clicked between the thing component (input) and the rules engine (output).
+
+(1) Input: Add a property to the thing
+```
+property = Property("variable_name", PropertyType.INT, 0)
+thing.add_property(property);
+```
+
+(2) Output: Add an action to the thing
+```
+thing.add_action("sameple_action")
+def custom_function(): ## define your action function
+    print("action")
+
+thing.sameple_action = custom_function
+```
+
+Register the sensor to the gateway
+```
+app.register(thing)
+```
+
+Connect the gateway to the server
+```
+app.start()
+```
+
+#### 3) disconnect gateway (python code)
+disconnect gateway
+```
+app.stop()
+```
+ 
+## API
+### Gateway
+#### Constructor
+```
+Gateway(name, deviot_server="deviot.cisco.com", connector_server="deviot.cisco.com:18883", kind="device", account="")
+```
+The Gateway() constructor takes the following arguments:  
+**name**  
+The unique name of a gateway  
+**deviot_server**  
+The address for the DevIoT server. The default value is 'deviot.cisco.com'. It does not need to add the protocol. The default protocol is HTTPS(secure connection).  
+**connector_server**  
+The address for the MQTT server. The default value is 'deviot.cisco.com:18883'. It does not need to add the protcol. The default protocol is TCP(unsecure connection) and the default port number is 1883.  
+**kind**  
+The kind of a gateway  
+**account**  
+Your DevIoT account. you also can use empty string, it will let all account get the data of your gateway.
+#### register()
+```
+register(*things)
+```
+The register() function adds things to the gateway. The thing should not have been already registered.  
+**thing**  
+A *Thing* instance to register
+#### load()
+```
+load(filename, class_directory=None)
+```
+The load() function registers things from an JSON file named [filename] and the custom Thing-sub classes inside [class_directory].  
+**filename**  
+The JSON file having information of things. filename should include its extension. The way to write this JSON file is described in **gateway-python-starter-kit**.  
+**class_directory**  
+The directory which has custom Thing-sub class. If the Thing-sub class is defined in the same directory, class_directory should be omitted or be None.
+
+#### deregister()
+```
+deregister(*things)
+```
+The deregister() function deletes things from the gateway.  
+**\*things**  
+A *Thing* instance to deregister
+#### update_thing()
+```
+update_thing(thing, **new_value)
+```
+The update_thing() function updates the values of a thing.  
+**thing**  
+The thing to be updated. It can be the string id of a thing or a Thing instance.  
+**\*\*new_value**  
+The keyword arguments for the updated values. The key is the name of properties and the value is the new value.
+
+#### start()
+```
+start()
+```
+Connect the things of the gateway to the DevIoT server and the MQTT server
+#### stop()
+```
+stop()
+```
+Disconnect the gateway from the DevIoT server and the MQTT server.
+### Thing
+#### Constructor
+```
+Thing(id, name, kind=None)
+```
+The Thing() constructor takes the following arguments:  
+**id**  
+the unique id of a thing  
+**name**  
+the display name of a thing in DevIot  
+**kind**  
+the kind of a thing
+
+#### add_property()
+```
+add_property(*thing_properties)
+```
+The add_property() adds properties to a Thing instance. When a string is used for arguments, new Property instance named the string is added to the Thing instance.  
+**\*thing_properties**  
+The list of Property instances or the string value of properties' name.
+#### add_action()
+```
+add_action(*thing_actions)
+```
+The add_action() adds actions to a Thing instance. When a string is used for arguments, new Action instance named the string is added to the Thing instance.  
+**\*thing_properties**  
+The list of Property instances or the string value of actions' name.
+
+### Property
+#### Constructor
+```
+Property(name, type=PropertyType.INT, value=None, range=None, unit=None, description=None)
+```
+The Property() constructor takes the following arguments:  
+**name**  
+The name of a property. It should be unique in a thing.  
+**type**  
+The variable type of a property. There are 4 types: int, bool, string, color. You can use PropertyType.INT, PropertyType.BOOL, PropertyType.STRING, PropertyType.COLOR after importing 'PropertyType' class from 'cisco_deviot.thing'.  
+**value**  
+The value of a property.  
+**range**  
+The range of a property's value.  
+**unit**  
+The unit of a property. It is string value.  
+**description**  
+The description for a property. It is shown at the page of each thing.
+
+### Action
+#### Constructor
+```
+Action(name, **kwargs)
+```
+The Action() constructor takes the following arguments:  
+**name**  
+The name of Action. The method with the same name should be defined in a class or an instance. So the name must not overlap with other methods of Thing class like *add_property* and *add_action, and the name cannot contain space ' '.  
+**\*\*kwargs**  
+The keyword arguments for properties in an action. Key is the name of a property and the value of the property. It is not mandatory.
+
+#### add_parameter()
+```
+add_parameter(action_parameter)
+```
+The *add_parameter* adds properties to an Action instance.  
+**action_parameter**  
+A Property instance to be added