Add common and useful netstat use case

This is one of the most useful netstat options I've found.

I thought it was worth sharing.

pages/linux/netstat.md

@@ -25,3 +25,7 @@
 - List routes and do not resolve IP to hostname:
 
 `netstat -rn`
+
+- List listening TCP and UDP ports (+ user and process if you're root):
+
+`netstat -lepunt`