document firewall rule for mqtt access

README.md

@@ -47,6 +47,7 @@ enable EPEL Repo:
 # dnf install firewalld
 # firewall-cmd --add-service={http,https} --permanent
 # firewall-cmd --remove-service=cockpit --permanent
+# firewall-cmd --add-port=1883/tcp --permanent   ; only for MQTT
 # firewall-cmd --reload
 
 List Rules: