Add mailing list and IRC channel

Creating this as a pull request just to notify other contributors - I've created a google group for announcements / asynchronous communication that doesn't fit into the pull request / github issue paradigm. I've also set up a freenode chat (#ruby-json-schema) for discussing issues in a live format as desired.

I'm not saying we need to use either of these, and don't expect either to be very active. However, I am going to begin formalizing the release process a bit more (release notes, etc) so that we can all follow a standard practice and provide a proper outlet (mailing list) for such announcements.

Let me know if you think this is crazy or not!