Update Readme to include allowing UDP port 3478

[danielfournier-tech]

I typically have server firewalls pretty locked down by default and spent a good bit of time looking at network logs to release that I needed to allow UDP 3478 and think it should be more clear in the documentation / readme. Thanks for making this cool project

[jmattheis]

Yeah, this should be included in the install page.

FYI: when using turn any udp port can be used for the turn relay, so every udp port should be allowed. The used udp port range can be restricted via the SCREEGO_TURN_PORT_RANGE setting.

[ajmandourah]

when using docker also in none host network configuration you must use udp for port 3478 otherwise it will not connect.