Code review guide

Based on our discussions earlier, I'm working on a code review guide. This documents the process and best practices from an Archivematica point of view - hoping to make this PR an open discussion.

The guide isn't finished, but I'm putting it up in this initial state so we can start discussion right away!

(The actual guide won't live in this repo; it's just made from here so there can be a PR to act as a discussion.)