docs: add Windows rsync installation requirement
docs: add Windows rsync installation requirement

Perhaps unsurprisingly, Git Bash doesn't come with rsync.

In the long run we'll need either a more precise installation method,
a replacement for rsync, or to go all-in on WSL; but this will
probably do for now.

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