add readme to explain how it works

readme.md

+# Errbot Matrix Backend
+
+*note:* This is currently a (functional) work in progress.
+
+## Features
+* Get message from matrix (rooms, private messages)
+* Send messages to matrix (responses) - with markdown support
+
+### Still todo
+Most (all) of these are in the library, just need mapping
+
+* General code tidy up
+* Some missing features for backend API (room methods, user methods)
+* Support for e2e encryption (the matrix library natively supports it)
+* Support for non-text message types (ie, images)
+* Support for reactions
+
+# Known issues
+
+## Strange setup
+The bot will exit if you use password authentication (after displaying the token and device id) - you then
+need to manually stick these in the config file. Would be better if the bot could cache this in some errbot-y
+way.
+
+If the bot doesn't do this, the bot generates a new device on each connection, which quickly becomes madness
+on the server-side (100s of devices when developing). By fixing the device ID and token, this doesn't happen.
+
+The configuration file for errbot looks like this:
+
+```
+backend = 'matrix'
+bot_extra_backend_dir = r'./backends/' # path to backends folder
+
+BOT_IDENTITY = {
+    'homeserver': 'https://matrix.fgmx.uk',
+    'username': '@errbot:fossgalaxy.com',
+
+    # set these to get the token and device id
+    'password': '', # not used if token is set
+    'device': '', # not used if token is set
+
+    # once known...
+    #'device_id': '', # DEVICE ID HERE
+    #'token': '' # ACCESS TOKEN HERE
+}
+```