Add a backwards compatibility guide

I recently discovered the [symfony backwards compatibility guide](http://symfony.com/doc/current/contributing/code/bc.html) and thought it was a great idea. We have previously had a pretty squishy idea of what backwards compatibility meant, and when/where breaking changes could be done. Since the information only lived in our heads, it was hard for new community members to find out about it, or understand the guidelines.

I haven't proofread this entirely, and figure there will be some discussion around what we want in/out of our BC guidelines so I can fix the issues in the resulting discussion.