Better docs

Taking what I've learned from pinging Eric while using WP_Mock for the first time, I've tried to re-work the docs to be a little more verbose. The goal was to make it accessible for developers who are a) new to WP_Mock or b) new(er) to testing in general, and perhaps heard "you should be testing, go try WP_Mock!" at a conference.

This also references #20, though that reads (to me) as a pretty open-ended ticket.