Add an example event listener, improve documentation, and add a commit discovery event

Summary: Improve documentation around Phabricator events.

Test Plan: Generated and read documentation. Ran test script.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran

Maniphest Tasks: T1092

Differential Revision: https://secure.phabricator.com/D2917