Add communication layer docs

Add breif description of the link, connector and messages communication
concepts implemented by lib/link.

docs/communication.md

@@ -0,0 +1,58 @@
+Communication Layer
+===================
+
+<sub>This document describes the communication layer in Clusterio and
+unless you're developing Clusterio it's probably not going to be very
+useful for you.</sub>
+
+Communication is facilitated by the lib/link library and is based on
+three core abstractions: links, connectors and messages.  Links
+represents one side of a two way communication pipe, connectors deal
+with connecting and establishing communication, and messages are the
+content sent over the link.
+
+
+Link
+----
+
+The Link class represents one side of a two way communication channel
+where the actual communication is done by a connector associated with
+the link.  The link also knows what the endpoint types it's connected to
+are via the source and target attributes.
+
+The endpoint types currently implemented are master, slave, instance,
+and control.  Although slave and instances run in the same Node.js
+program they use virtual links to communicate between them in order to
+simplify the communication architecture.
+
+Upon creation the link will register with the connector in order to
+receive and process messages from it.  The messages received are
+validated using the validator registered for the type of the message.  A
+message for which there is no validator for is considered an error.
+
+
+Connector
+---------
+
+The connector used with a link is responsible for establishing and/or
+maintain the connection that messages are sent over.  Reconnect logic is
+expected to be implemented in the connector, though this has not yet
+been done.
+
+There's also the virtual connector which lets two links in the same
+program be connected to each other without having to establish a
+network connection.
+
+
+Message
+-------
+
+There's two types of messages currently implemented apart from the
+Link builtin close message and that's Request and Event.  The builtin
+messages are defined in [lib/link/messages.js](../lib/link/messages.js)
+and are attached to links via the attachAllMessages function, which is
+usally called in the Link subclass constructor.
+
+The messages define which links they are valid on and only attaches
+handlers for those links.  There's also a forwarding mechanism for when
+a request or event is to be forwarded another hop.