:memo: Unify the two contributing documents

Both documents had some really great stuff to them, but there was too much
overlap. I did my best to unify them into one document, taking the best
features of each.
Split out the package-specific information

Per PR feedback, splitting out package-specific development information
into its own document.
Merge pull request #2295 from lee-dohm/contributing

:memo: Unify the two contributing documents