Login, read receipts, and typing notification docs for iMessage on nosip

[cdknight]

When I was setting up the Mautrix iMessage bridge for nosip I encountered some difficulties getting typing indicators and puppeting to work due to a lack of documentation. Anyway, I wanted to document these things since hopefully it would make them easier for future users setting up this bridge.

My PR just adds a couple of points on how to enable read receipts and that you need the custom puppet enabled if you want typing indicators to work. It also has documentation about how to log in for puppeting, but I'm not sure if the login-matrix stuff is right.

I'm not too sure about the contribution guidelines to this repo/the validity of what I added, so feel free to reject this or comment on any changes that need to be made.

Thanks!

[tulir]

This doesn't really belong in iMessage docs at all, double puppeting is already documented in https://docs.mau.fi/bridges/general/double-puppeting.html

[cdknight]

@tulir I wasn't aware that the double puppeting page exists (I see it has a lot more detail and is informative to me, actually). As someone who came to the documentation knowing nothing about bridges/puppeting, mayhaps it would be helpful to link the double-puppeting page on the nosip documentation page (just because ideally as an end user I would expect this information to be briefly mentioned, or maybe I should read through the entire docs next time)?