adding append_to_user_agent docs

Author: maxkahan

README.rst

@@ -669,6 +669,15 @@ to all clients connected to session or to a specific connection:
   # To a specific connection
   opentok.play_dtmf(session_id, connection_id, digits)
 
+Appending to the User Agent
+-------
+
+You can append a string to the user agent that is sent with requests:
+
+.. code:: python
+
+  opentok.append_to_user_agent('my-appended-string')
+
 Samples
 -------