Add a note about publishing haproxy's input port to the host

This has been mentioned in the comments twice, so I thought I'll add it for people to save them time wondering why their port configuration doesn't seem to be working :)

haproxy/README.md

@@ -56,6 +56,8 @@ $ docker run -it --rm --name haproxy-syntax-check haproxy:1.5 haproxy -c -f /usr
 $ docker run -d --name my-running-haproxy my-haproxy
 ```
 
+You may need to publish the ports your haproxy is listening on to the host by specifying the `-p` option, for example `-p 80:80` to publish port 80. Make sure the port you're using is free.
+
 ## Directly via bind mount
 
 ```console