Create README.md

Added README for UFW plugin issue: [Good first PR] Add READMEs to undocumented plugins #7175

plugins/ufw/README.md

+# UFW
+This plugin adds support for managing everybody's favorite Uncomplicated Firewall (UFW). Learn more about [`UFW`](https://wiki.ubuntu.com/UncomplicatedFirewall).
+
+To use it, add ufw to the plugins array of your zshrc file:
+```
+plugins=(... ufw)
+```
+UFW is a simple Ubuntu interface for managing iptables. 
+
+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