docs/style: On what to cover in commit messages.
docs/style: On what to cover in commit messages.

Based in part on a series of comments I made on #2993, and on:
  https://github.com/zulip/zulip-mobile/pull/3292#issuecomment-495288790

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile