This started as a PSA in the form of a series of chat messages in
`#general` on chat.zulip.org; putting them here, with some editing,
to make their value more durable.
Also rearrange this doc slightly so that it's not specific to
the server codebase, except in a few explicit spots.
The bit that's for authors should probably be somewhere else.
I think there isn't right now a great natural spot for it --
probably the top of docs/git-guide, some parts of docs/version-control,
and that paragraph here should all turn into a top-level "guide
to submitting code to Zulip" doc, which would link to the rest
of docs/git-guide and to some other resources. Leaving that
for another day.