Updates README.md with message_id

Author: anglinb

README.md

@@ -95,7 +95,7 @@ always be the login of the user typing the command, and `room_id` will be where
 it was typed.
 The optional `mention_slug` parameter will provide the name to use to refer to
 the user when sending a message; this may or may not be the same thing as the
-username, depending on the chat system being used.
+username, depending on the chat system being used. The optional `message_id` parameter will provide a reference to the message that invoked the rpc.
 
 You can return `jsonrpc_success` with a string to return text to chat. If you
 have an input validation or other handle-able error, you can use