Update protocol-description.md

Author: anglinb

docs/protocol-description.md

@@ -44,6 +44,7 @@ invocation. A method invocation is a JSON object with the following fields:
 
  * `user`: A slug username corresponding to to the command giver's GitHub login.
  * `mention_slug`: Optional. If provided, a string which should be used to mention the user when sending a message in response. For example, Slack requires that users be mentioned using user IDs instead of usernames.
+ * `message_id`: Optional. If provided, an id that uniquely identifies the message that generated the CRPC call. Useful for linking back to the original command to provide context.
  * `room_id`: A slug room name where the command originated.
  * `method`: The method name, without namespace, of the matching regex.
  * `params`: A mapping of parameter names to matches extracted from named capture groups in the command's regex. Parameters that are empty or null should not be passed.