cat /dev/brain > CONTRIBUTING.rst

This is a bunch of thoughts on conventions that we've been adding organically through the sample-writing process, mostly examples and clarifications of things that I was looking to correct in my consistency passes the other day. If it makes my OCD twitch, it's probably going to make someone else's, after all!

@rdodev, @kenperkins, @ycombinator, @rgbkrk: Let me know if you think these are reasonable guidelines to strive for. I know we have quite a few samples written that don't conform; I'll do my best to touch up existing ones periodically as we go.