docs: add a doc about Channel

docs/channel.md

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+# Channel
+
+A Channel is a class that abstracts an object that exchanges data with the outside like a TCP socket. In addition to TCP, it can support functions such as UDP sockets and Unix domain sockets.
+In order to handle events that occur in Channel, el uses ChannelPipeline, ChannelHandlerContext, and ChannelHandler.
+
+![channel-pipeline-diagram](../images/docs-arch-channel-pipeline.png)
+
+Each Channel has a ChannelPipeline. ChannelPipeline manages ChannelHandler functions, and calls the functions when events occur. ChannelPipeline uses ChannelHandlerContext to manage ChannelHandlers by creating an doubly linked list.
+
+## ChannelPipeline
+
+When a Channel receives an event(like receiving data from a network), it passes it to a ChannelPipeline. The ChannelPipeline calls the first handler. When each handler finishes handling the event, it calls the next handler.
+
+If we send a message to a TCP channel, the invocation order of the handlers is reversed. 
+
+---
+TODO: Add an image that explains the order of invocation of handlers.
+---
+
+## ChannelHandler
+
+A ChannelHandler is an event listener that listens events from a Channel. A ChannelHandler can listen all types of event from a Channel at a time. You can use the ChannelHandler to parse a message, to log the message, or to write business logic.
+
+## ChannelHandlerContext
+
+A ChannelHandlerContext is a wrapping class who is used by a ChannelPipeline to manage ChannelHandlers. 
+
+## LocalChannel
+
+A LocalChannel is a Channel that should be used only for test purpose. We can connect two LocalChannel in a process's memory. We cannot use the LocalChannel to communicate with other process or other machine.