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'm wondering if `preventDefault` is actually the correct name here. That makes sense in the context of a browser having a "default action" for a bubbled DOM event, but for us a simpler word like `cancel` seems more appropriate.