Give example of generating nested/dynamic segment using Ember CLI

[jenweber]

https://guides.emberjs.com/v3.0.0/routing/defining-your-routes/#toc_dynamic-segments

Here, we talk about dynamic segments but don't say how to create the necessary files and router entry. If you try to do it by hand, it's a bad time. Add an example ember cli command to this section of the guides.

Note, this issue is set aside for the EmberConf contributors workshop on 3/12. If it's not claimed by the end of that day, feel free to grab it!