API guidelines doc

This is super in-progress. A lot of this information is sort of tribal knowledge, but I wanted to get it in out in the open and us on the same page.

This should give us somewhere to look when we're like "how should I name that method/event?", or "how do I doc a method that takes a callback that takes an options hash."
I agree on the `preventDefault` thing, I was going to mention something about that but my brain got stuck on the future/present/past tense event naming stuff.

Ok, just `will-/did-x` where `will-x` is cancelable makes sense.