Broken crypto link in the readme

[babolivier]

The readme links to https://docs.mau.fi/python/latest/api/mautrix.crypto.html, which shows a 404 error page.

I'm opening an issue rather than fixing it myself because I'm not sure where it should point, it looks like mautrix does have a mautrix.crypto module but I can't see a documentation page for it.

[tulir]

I only recently got around to figuring out how to sphinx properly, but haven't generated docs for all the modules yet.