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 into Spree, the authorization code contained in Spree::Admin::BaseController is rather obfuscated and will lead any developer down a bunny trail when creating custom roles and controllers that form part of Spree's admin panels.

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.