[RFC] Updating CONTRIBUTING.md where @ashleyh left off

I'm not quite sure where @ashleyh went off to, but #288 hasn't been updated in over a month.

And based on some of the PRs that have been received and some things that have been ran into (#680), I think it's important to get ashleyh's changes into master.

After we update CONTRIBUTING.md, I can address the broad style changes like what @philix mentioned in #680.

I also added a section on Commit messages because it was there originally but was removed by ashleyh's changes.

Thanks all.