[docs] Small addition to the Express Readme Part (socketio#2846)

Readme.md

@@ -113,7 +113,7 @@ io.listen(3000);
 Starting with **3.0**, express applications have become request handler
 functions that you pass to `http` or `http` `Server` instances. You need
 to pass the `Server` to `socket.io`, and not the express application
-function.
+function. Also make sure to call `.listen` on the `server`, not the `app`.
 
 ```js
 var app = require('express')();