Add deprecation info in a docstring

Author: sblondon

matrix_client/client.py

@@ -341,7 +341,7 @@ def join_room(self, room_id_or_alias):
         return self._mkroom(room_id)
 
     def get_rooms(self):
-        """ Return a dict of {room_id: Room objects} that the user has joined.
+        """ Deprecated. Return a dict of {room_id: Room objects} that the user has joined.
 
         Returns:
             Room{}: Rooms the user has joined.