more text in the readme

README.md

@@ -25,13 +25,18 @@ Usage: tcpprism [arguments]
     -h, --help                       Show this help
 ```
 
-Eg.:
-
-```
+If you for example would like to mirror the traffic currently coming in to a
+server on port 80, to another server also running on port 80, but without
+stopping and changing port of the original server, you can use trick with
+iptables that redirects port 80 to the `tcpprism` server, which in turn will
+forward the traffic to both the original server and the server on the other
+host.
+
+```sh
 ulimit -n 16384
-tcpprism --listen 0.0.0.0:80 --forward 127.0.0.1:8080 --mirror 10.0.0.2:8080
+iptables -t nat -A PREROUTING -i ens5 -p tcp --dport 80 -j REDIRECT --to-port 8080
+tcpprism --listen 0.0.0.0:8080 --forward 127.0.0.1:80 --mirror 10.0.0.2:80
 ```
-
 ## Contributing
 
 1. Fork it (<https://github.com/84codes/tcpprism/fork>)