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."
@lee-dohm:

> Should the "something" in step two be "rolled back"? Or should we say that one can only cancel things from the will-x event? Once the x starts, it is going to complete unless there's an error.

I think there would only be `will-x` and `did-x`. The `will-x` event is cancelable, but not `did-x`. I'd like to think we can get away with not informing preceding event handlers that the action was cancelled. I can't think of a way to do it that's not fairly ornate. Perhaps in situations where it's needed the object could emit an `x-cancelled` event as well.