Add information about Roles in the Admin namespace.

Per a IRC discussion with @radar, I have included a section in the Spree guides about how roles interplay with Spree's Admin panels.

While it might not be a very common scenario where someone will be integrating custom roles and custom controllers in to Spree, the code that runs by the Admin BaseController is rather obfuscated and will lead any developer down a bunny trail.

After going down that bunny trail and spending several hours of debugging, I've added a section in the guides to hopefully aid users looking to implement custom roles and custom controllers in Spree's administrative section.