add docc root page

Author: alexdeem

README.md

@@ -17,6 +17,10 @@ Add `.package(url: "https://github.com/SwiftScream/swift-environment-decoder.git
 
 ## Usage
 
+[Detailed documentation](https://swiftpackageindex.com/SwiftScream/swift-environment-decoder/master/documentation/environmentdecoder) is available on the swift package index.
+
+This should be enough to get you started...
+
 ### Decoding the Environment
 
 ```swift

Sources/EnvironmentDecoder/EnvironmentDecoder.docc/EnvironmentDecoder.md

@@ -0,0 +1,154 @@
+# ``EnvironmentDecoder``
+
+
+## Overview
+
+Suppose you're builing a server app and want to enable specifying the port to bind to in the environment. You might write something like this:
+
+```
+let port = Int(ProcessInfo.processInfo.environment["PORT"]) ?? 3000
+```
+
+As your use of the environment grows, this can become cumbersome and error prone.
+
+``EnvironmentDecoder`` enables type-safe access to the environment leveraging Swift's `Decodable`.
+
+To use it you specify the shape of the environment that your application expects as a `Decodable` struct. ``EnvironmentDecoder`` can instantiate an instance of this struct from the current environment, throwing an Error if the environment is not valid.
+
+## Example
+
+Let's say you have a server app and want the environment to specify the port to bind to, a list of allowed regions, and a couple of boolean feature flags.
+
+Your environment might look like:
+
+```
+PORT=1234
+ALLOWED_REGIONS=regionA,regionB,regionC
+FEATURE_FLAGS_ENABLE_A=true
+FEATURE_FLAGS_ENABLE_B=false
+```
+
+``EnvironmentDecoder`` enables access to this like:
+ 
+```swift
+import EnvironmentDecoder
+
+struct MyEnvironment: Decodable {
+    let port: UInt16
+    let allowedRegions: [String]
+    let featureFlags: MyEnvironmentFeatureFlags
+}
+
+struct MyEnvironmentFeatureFlags: Decodable {
+    let enableA: Bool
+    let enableB: Bool
+}
+
+let environment = try EnvironmentDecoder().decode(MyEnvironment.self)
+```
+
+## Mapping Properties to Environment Variables
+
+Environment variables are a flat namespace, but Decodable types can be nested to create meaningful hierarchy.
+
+``EnvironmentDecoder`` will, by default, leverage the type hierarcy to create meaningful environment variable names. This can be disabled in the initializer by specifying `prefixKeysWithCodingPath: false`.
+
+#### An Example:
+
+```swift
+struct DatabaseConnection: Decodable {
+    let connectionString: String
+}
+
+struct WebServiceConnection: Decodable {
+    let host: String
+    let port: Int16
+}
+
+struct Environment: Decodable {
+    let region: String
+    let database: DatabaseConnection
+    let webService: WebServiceConnection
+}
+```
+
+When `prefixKeysWithCodingPath` is enabled the environment is expected to be:
+```
+REGION
+DATABASE_CONNECTION_STRING
+WEB_SERVICE_HOST
+WEB_SERVICE_PORT
+```
+
+When `prefixKeysWithCodingPath` is disabled the environment is expected to be:
+```
+REGION
+CONNECTION_STRING
+HOST
+PORT
+```
+
+The usual methods that the Codable framework provides can be used to customise coding keys.
+
+## Supported Data Types
+
+Almost all `Decodable` data structures are supported.
+
+The one limitation is that elements of an Unkeyed Container must not be represented by an Unkeyed Container or Keyed Container.
+For example, elements of an array cannot themselves be an array, or an object represented by key/value pairs.
+
+It is possible to use a custom Decodable type in an array, so long as that type decodes from a single value.
+
+Outside of this limitation, all Decodable types are supported. Some notes on specific data types follows.
+
+#### Data
+
+Several strategies are supported for decoding `Data`.
+
+Refer to ``EnvironmentDecoder/DataDecodingStrategy``
+
+#### Date
+
+Several strategies are supported for decoding `Date`.
+
+Refer to ``EnvironmentDecoder/DateDecodingStrategy``
+
+#### Unkeyed Containers
+
+UnkeyedContainers, for example Arrays, are supported so long as the element value is represented by a single value.
+
+UnkeyedContainer values are decoded from a character-separated list of values.
+By default the separator character is a comma. This can be customised by specifying `unkeyedContainerSeparator` in the initializer.
+
+By default whitespace is trimmed from unkeyed container values. This can be disabled by specifying `trimWhitespaceFromUnkeyedContainerValues` in the initializer. 
+
+
+## Error Handling
+
+If the environment cannot be decoded into the specified type, a `Swift.DecodingError` is thrown.
+
+This is a great way to catch issues with the environment early. In some cases it may be appropriate to output a message, and terminate the process.
+
+Some examples of why this may occur:
+
+ - a required environment variable is not specified
+ - the value of an environment variable cannot be decoded as the specified type
+
+```swift
+let environment: MyEnvironment
+do {
+    environment = try EnvironmentDecoder().decode(MyEnvironment.self)
+} catch {
+    fatalError(error.localizedDescription)
+}
+```
+
+## Alternative Environment
+
+By default EnvironmentDecoder will decode `ProcessInfo.processInfo.environment`.
+If that's not what you want, you can pass in a `[String: String]` instead.
+
+```swift
+let someEnvDictionary: [String: String] = //...
+let environment = try EnvironmentDecoder().decode(MyEnvironment.self, from: someEnvDictionary)
+```