Add links to HipChat rooms

Specifically:

1. From http://hibernate.org/community to https://www.hipchat.com/gMTG8dBAq (Hibernate User room). Also link to a page on hibernate.org from that room discussing the caveats for that HipChat room (no history for guests, how to register, waiting for replies, etc)
2. From http://hibernate.org/orm/contribute/ to https://www.hipchat.com/goMzVNYC9 (ORM Dev room)