ufw: add README (#8227)

plugins/ufw/README.md

+# UFW plugin
+
+This plugin adds completion for managing everybody's favorite Uncomplicated Firewall (UFW),
+a simple interface for managing iptables. Learn more about [`UFW`](https://wiki.ubuntu.com/UncomplicatedFirewall).
+
+To use it, add ufw to the plugins array of your zshrc file:
+```
+plugins=(... ufw)
+```
+
+Some of the commands include:
+
+* `allow <port>/<optional: protocol>` add an allow rule 
+* `default` set default policy
+* `delete <port>/<optional: protocol>` delete RULE
+* `deny <port>/<optional: protocol>` add deny rule
+* `disable` disables the firewall
+* `enable` enables the firewall