add an example to readme

We need some kind of minimalistic usage example, so when people start to use this module, they'll have something working to start with.

I was trying to use `http.createServer(Router()).listen(3000)` at first, which lead to an ugly runtime error. So I wanted to save someone a trouble of debugging this.

Code is copied from tests. I wonder if it's possible to get rid of finalhandler there for simplicity sake.