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."
Talking it out with myself, I like the `will-x`, `x` and `did-x` pattern also. No matter what is decided, adding the feature to Emissary is probably a good idea.