Add some information on registering AS's

CHANGES.rst

@@ -40,8 +40,10 @@ Application services:
   instead their configuration should be saved to a file and listed in the
   synapse ``app_service_config_files`` config option. The AS configuration file
   has the same format as the old ``/register`` request.
+  See `docs/application_services.rst`_ for more information.
 
 .. _`docs/postgres.rst`: docs/postgres.rst
+.. _`docs/application_services.rst`: docs/application_services.rst
 .. _`Registration`: https://github.com/matrix-org/matrix-doc/blob/master/specification/10_client_server_api.rst#registration
 .. _`Retrieving Server Keys`: https://github.com/matrix-org/matrix-doc/blob/6f2698/specification/30_server_server_api.rst#retrieving-server-keys
 .. _`Application Services`: https://github.com/matrix-org/matrix-doc/blob/0c6bd9/specification/25_application_service_api.rst#home-server---application-service-api

docs/application_services.rst

+Registering an Application Service
+==================================
+
+The registration of new application services is implementation dependent. In
+synapse you need to create a new configuration file for you AS and add it to
+the list of AS's specified under ``app_service_config_files`` synapse
+config option.
+
+The format of the AS configuration file is as follows::
+
+    url: <base url of AS>
+    as_token: <token AS will add to requests to HS>
+    hs_token: <token HS will ad to requests to AS>
+    sender_localpart: <localpart of AS user>
+    namespaces:
+      users:  # List of users we're interested in
+        - exclusive: <bool>
+          regex: <regex>
+        - ...
+      aliases: []  # List of aliases we're interested in
+      rooms: [] # List of room ids we're interested in
+
+See the spec_ for further details on how application services work.
+
+.. _spec: https://github.com/matrix-org/matrix-doc/blob/master/specification/25_application_service_api.rst#application-service-api
+