development.7/FILES: add CONTRIBUTING + git-arc.sh

MFC after: 3 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1300

(cherry picked from commit c2c6f08c8b47cfda79529e0a1016b8f7803c0ec8)
(cherry picked from commit 29020ea39d6c55983dfa49b3c2928acdd851c1d3)