Add a small "contributing" file, for GitHub.
Use MD links
Remove references to the numba-dev mailing-list, replace with numba-users
Merge pull request #791 from pitrou/contributing

Fix #787: add succinct contribution guidelines
Merge pull request #783 from pitrou/cmath_support

cmath module support
Merge branch 'master' of github.com:numba/numba into test_support