Added Sphinx based documentation

This patch adds the docuemntation source tree in docs.

You can build the documentation with

tox -edocs

View the documentation by opening a browser and viewing
aprsd/docs/_build/index.html

docs/plugin.rst

@@ -0,0 +1,55 @@
+APRSD Command Plugin Development
+================================
+
+APRSDPluginBase
+------------------------
+
+Plugins are written as python objects that extend the APRSDPluginBase class.
+This is an abstract class that has several properties and a method that must be implemented
+by your subclass.
+
+Properties
+----------
+
+* name - the Command name
+* regex - The regular expression that if matched against the incoming APRS message,
+          will cause your plugin to be called.
+
+Methods
+-------
+
+* command - This method is called when the regex matches the incoming message from APRS.
+            If you want to send a message back to the sending, just return a string
+            in your method implementation.  If you get called and don't want to reply, then
+            you should return a messaging.NULL_MESSAGE to signal to the plugin processor
+            that you got called and processed the message correctly.  Otherwise a usage
+            string may get returned to the sender.
+
+
+Example Plugin
+--------------
+
+There is an example plugin in the aprsd source code here:
+aprsd/examples/plugins/example_plugin.py
+
+.. code-block:: python
+
+    import logging
+
+    from aprsd import plugin
+
+    LOG = logging.getLogger("APRSD")
+
+
+    class HelloPlugin(plugin.APRSDPluginBase):
+        """Hello World."""
+
+        version = "1.0"
+        # matches any string starting with h or H
+        command_regex = "^[hH]"
+        command_name = "hello"
+
+        def command(self, fromcall, message, ack):
+            LOG.info("HelloPlugin")
+            reply = "Hello '{}'".format(fromcall)
+            return reply