Please add troubleshooting instructions

[rombert]

Hello and thanks for the great project! As a Linux user, I am really happy to not have to rely on the Android app to control my key light.

There were some things I needed to figure out to make keylightctl work, and it would be if some troubleshooting notes could be added to the README. Here's a summary of my notes:

1. If keylightctl discover does not find your devices, you need to make sure that your firewall allows mdns. For firewalld, use sudo firewall-cmd --list-all and check that 'mdns' is included in the 'services' listing. If not, add it with sudo firewall-cmd --add-service mdns
2. If your devices are added but describe and switch fail with errors such as dial tcp: lookup elgato-key-light-XXXX.local.: no such host, then your resolver may not support mDNS. If using systemd-resolve, you need to manually configure global mDNS resolution and and per-interface mDNS resolution, see for instance https://unix.stackexchange.com/questions/459991/how-to-configure-systemd-resolved-for-mdns-multicast-dns-on-local-network .