feat(docs): Add some basic API documents.

@zaach, @6a68, @nchapman - could I get a review of these? I'm adding @6a68 and @nchapman because they need some docs NOW.
Unrelated, but it may be nice if the `release` task also updates the CONTRIBUTORS or AUTHORS file (for example: see `npm run contributors` which calls our custom script in package.json).