Add note for firewall when no ssdp/udp pakets are received

[mStirner]

Perhaps Firewall related issue, like on my test system.
No ssdp message is received when firewall is enabled.

Even with socat:

socat UDP4-RECVFROM:1900,ip-add-membership=239.255.255.250:0.0.0.0,fork,reuseaddr -

After disabling the firewall, it works.
OS: CentOS 8 Stream

Something to read:
https://unix.stackexchange.com/questions/19060/fedora-firewall-with-upnp
https://unix.stackexchange.com/questions/625969/upnp-with-firewalld

TODO:

• Test on local prod system (Rocky Linux 9.1, firewall disabled)
  Same shit, as on test
• Document firewall rule to fix this
  firewall-cmd --zone=public --add-port=1900/udp

[mStirner]

https://unix.stackexchange.com/questions/732565/ssdp-udp-multicast-on-fresh-installation-not-working