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 definitely like the consistency of `will-` and `did-`. We could probably add a feature to Emissary to allow events to be deprecated and warn people at subscription time.