ActiveAdmin: Define overwritable resource inside an engine

I do not take all of the credit for this. The ActiveAdmin wiki already had a page describing how to define a resource in an engine. But the code in the wiki page would not allow the new resource defined in the engine to be overwritten by the main application. I updated the wiki page with code and an explanation to allow the resource to be overwritten by the main application.

https://github.com/gregbell/active_admin/wiki/Define-a-resource-inside-an-engine