Print message on stderr when no routes are defined.

Newbies get confused when they run 'rake routes' and no routes are defined. Let's
point them to some documentation.

Thanks to @burtlo for the suggestion.

```
$ rake routes
You don't have any routes defined!
Please add some routes in config/routes.rb.
For more information about routes, see the Rails Guide: http://guides.rubyonrails.org/routing.html
```

![routes](https://f.cloud.github.com/assets/27786/168732/18a92f1e-7a06-11e2-9a54-1689d14dad2a.png)