Add instructions for users with Firewall enabled.
### Proposed Change

I just installed homebridge on my new homelab running Fedora Server.  Everything smooth as expected until it was time to add the bridge to the iOS Home App.   After scanning the QR code the Home App was unable to connect back to homebridge.

I've had issues with other containerized apps due to Fedora's "deny everything" firewall (which I appreciate) so I added port 51459 to my firewall rules and the process completed smoothly.

I'm not sure its always going to be port 51459 that needs to be open (not sure about bonjour protocol) - but I found it out by looking at the JSON config file under settings.

Hope this helps, and thanks for the awesome work!

### Wiki Page Link

https://github.com/homebridge/homebridge/wiki/Install-Homebridge-on-Docker