Update documentation, separate out full changelog

[tharsheblows]

trafficstars

Fixes #1565

This fixes the issue about the changelog in readme.txt being too long in the WordPress plugins repo.

It adds changelog.md so that it's easy to link to on GitHub as well as on the repo -- I added a link to it at the bottom of the changelog in readme.txt.

I also removed the "Past contributors" section and replaced it with a link to the GitHub contributors page. This wasn't a linked list so I thought it would be easier to keep up to date correctly by linking to GitHub.