Add note about mxid to rt username mapping

README.md

@@ -4,6 +4,7 @@ A [maubot](https://github.com/maubot/maubot) for [RequestTracker](https://bestpr
 ## Features
 - Responds with some information about tickets if it sees RT ticket numbers in a given format
 - Implements basic functionality for interacting with RT
+- Assumes that the mxid localpart is equal to the RT username
 - Uses the REST 1.0 interface
 - Tested with `request-tracker4` on Debian