Update CONTRIBUTING and PROJECTS

This updates CONTRIBUTING to reflect the changes to use the
master branch and make explicit recommendations about updating
CHANGELOG.md and providing test coverage.  Also, this includes
some old updates to PROJECTS.md that I wrote to expand the list
of projects, pulling in things from old feature requests.
missing paren
Merge pull request #2434 from libgit2/rb/contributing-updates

Update CONTRIBUTING and PROJECTS