Add nested this.route to router docs

Added section describing usage of nested `this.route`

![screen shot 2014-09-23 at 1 00 30 pm](https://cloud.githubusercontent.com/assets/12481/4377977/f5bb0d60-4353-11e4-92ba-9a278ec31d0d.png)

@mixonic Double checking.. I believe that's what the routes output on my ember extension