Added some more stuff to contributing.md

Should close #147

We can talk about this some more, but in general I think we just need to capture some conventions and requirements for making the site work. The one thing I added, which has become second nature to me, is the practice of squashing commits on branches. It's not bad once you're used to it, and it really does keep the history cleaner, which is only going to help us with change logs going forward.