conference-protocol: add recording state

jami-daemon#699

Change-Id: Ic4a915c201dc4bc9061c14181abc20429b73ed47

developer/conference-protocol.md

@@ -80,6 +80,7 @@ Possible keys are:
 + isModerator = if it's a moderator
 + handRaised = if the hand is raised
 + voiceActivity = if the stream has voice activity
++ recording = if the peer is recording the conference
 
 ### New API