Add basic docs for Audio API, fixes #177.

docs/api/audio.rst

@@ -0,0 +1,19 @@
+.. _audio-api:
+
+*********
+Audio API
+*********
+
+The audio API is the interface we have built around GStreamer to support our
+specific use cases. Most backends should be able to get by with simply setting
+the URI of the resource they want to play, for these cases the default playback
+provider should be used.
+
+For more advanced cases such as when the raw audio data is delivered outside of
+GStreamer or the backend needs to add metadata to the currently playing resource,
+developers should sub-class the base playback provider and implement the extra
+behaviour that is needed through the following API:
+
+
+.. autoclass:: mopidy.audio.Audio
+    :members:

docs/api/index.rst

@@ -9,5 +9,6 @@ API reference
     models
     backends
     core
+    audio
     frontends
     listeners

mopidy/audio/__init__.py

@@ -208,6 +208,8 @@ class Audio(ThreadingActor):
         """
         Call this to deliver raw audio data to be played.
 
+        Note that the uri must be set to ``appsrc://`` for this to work.
+
         :param capabilities: a GStreamer capabilities string
         :type capabilities: string
         :param data: raw audio data to be played