Bug #1328716: Clean up CONTRIBUTING a bit.

Per Bug #1328716, here's a patch that updates CONTRIBUTING to mesh better with README.  It also fills in some gaps, particularly with tools, and makes the workflow more clear.  Given that working on this patch is my first exposure to the workflow I have no illusions as to perfect accuracy.  So feedback would be much appreciated.
The problem with this is that sometimes the revs we use (specified in dependencies.tsv) are out of sync, and so a blind `go get` may fail.

I'd suggest changing that to `go get -d`, add a comment about why, and a link to the "Dependency management" section.