contributing: add git/release info
contributing: add git/release info

Revamps the "Git practice" section to make it less of a git and GitHub
tutorial and more of a guide of expected contribution style.  In
particular, it answers the questions of:

* what tree contributors should branch from
* how to submit contributions
* how to structure commits
* how to format commit descriptions
* what contributions are considered acceptable
* what our expectations are around committing pills

Additionally, adds a "Releases" section that describes how we go about
making and deploying releases.

jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit