Get local user ID (#495)

* Get local user ID

How to get the id Participate local.

* Correction of the sequence of numbers

doc/API.md

 
 2. leave() - leaves the conference. Returns Promise.
 
+3. myUserId() - get local user ID.
+
 4. getLocalTracks() - Returns array with JitsiTrack objects for the local streams.
 
 5. addEventListener(event, listener) - Subscribes the passed listener to the event.