Add docs for firewalld-cmd banning

docs/banning.md

+# Banning IP Addresses for abuse
+
+```
+firewall-cmd --add-rich-rule="rule family='ipv4' source address='1.1.1.1' reject"
+```
+
+```
+firewall-cmd --add-rich-rule="rule family='ipv6' source address='1:2:3:4:6::' reject"
+```
+
+Note that on Gitlab, you must block the ip address for the docker zone:
+
+```
+firewall-cmd --add-rich-rule="rule family='ipv4' source address='1.1.1.1' reject" --zone=docker
+```
+
+To see the bans/rules:
+
+```
+firewall-cmd --list-all
+```
+
+To remove a banned IP Address:
+
+```
+firewall-cmd --remove-rich-rule='rule family="ipv6" source address="1:2:3:4:6::" reject'
+```