chore(COMMIT-MSG): Add Git COMMIT-MSG hook for validating commit message

Installation:
>> cd <repo>
>> ln -s ../../validate-commit-msg.js .git/hooks/commit-msg

See https://docs.google.com/document/d/1rk04jEuGfk9kYzfqCuOlPTSJw3hEDZJTBN5E5f1SALo/edit