Clarify room setup for HipChat users

It's non-intuitive, but you can't put the room name as the value for the room, you need to find the HipChat JID of the specific room and use that as the value of the repo.

I ran into this issue today when trying to setup this plugin with our HipChat setup, which prompted me to go down a rabbit hole debugging the handler itself, when all along the issue was the name of the room. I hope this helps others using HipChat.