[documentation] specify what mqtt version & protocol are available

[Neon-44]

Hey there. I would like to use ovms with mqtt via websocket (with TLS) (wss).

But I can't find anywhere whether ovms supports. It only says generically "mqtt", not specifying what version (3.1.1 or 5) or protocol (over TCP or over WebSocket).

I would be happy if someone could enlighten me (and possibly put it in the readme for future interested people?)