Mention support JID so can add to roster if non-roster messages blocked
Some users/servers block messages from JIDs not in the user's roster. During the registration process, the jmp-register support JID should be mentioned, so that the user can add it to their roster and retry the verification if it didn't work the first time.
It's possible that we don't want to support this mode, since it will making using JMP difficult for the user later (since they won't get messages from new phone numbers before first adding them to their roster), but possibly some users will be ok with that.
This is related to #11 (closed), as the "configuration issue on the client end" was that JIDs not in the user's roster had their messages blocked.