Add basic contributing content

Fixes #1611

A draft contributing document was merged a bit early, so I fleshed it
out to cover some of the basics.
Add section to contributing about Unit Tests
Merge pull request #1 from maurizi/patch-1

Add section to contributing about Unit Tests
Add JS testing section to CONTRIBUTING.md
Merge pull request #1613 from jwalgran/topic/add-basic-contributing-content

Add basic contributing content