Document exposing VNC port securely via SSH

[pascalw]

Hey @pl-semiotics thanks for this fantastic piece of software. It absolutely crushes https://github.com/rien/reStream in terms of latency and throughput, great work!

I noticed you recommend restricting the VNC port with iptables. I'm not sure if iptables even runs on the Remarkable (not out of the box at least). Fortunately libvncserver allows to bind on localhost only, which combined with an SSH tunnel allows to securely connect to the VNC server without exposing it publicly. I don't observe any increased latency running this way, it's still lightning fast (even via WiFi)!

Perhaps this can be mentioned in the README.

[pl-semiotics]

Thanks, @pascalw (sorry for the delay---I've been busy with non-rM work)! I entirely forgot that one could just pass arguments to libvncserver like that, honestly. This looks wonderful---I'll merge it later today or over the weekend.