docs: Add pointers for supplemental Flow "documentation".
docs: Add pointers for supplemental Flow "documentation".

Ray discovered this "cheat sheet" and mentioned it here:
  https://github.com/zulip/zulip-mobile/pull/3652#discussion_r338226403

Seems like something I'll want to find again.  Also take the
opportunity to write down advice about its issue tracker which I've
mentioned in more-ephemeral places before.

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