Added some context for rack apps, linked to the Rails on Rack guide.

railties/guides/source/getting_started.textile

@@ -90,7 +90,7 @@ Action View manages the views of your Rails application. It can create both HTML
 
 h5. Action Dispatch
 
-Action Dispatch handles routing of web requests and dispatches them as you want, either to your application or any other Rack application.
+Action Dispatch handles routing of web requests and dispatches them as you want, either to your application or any other Rack application.  Rack applications are a more advanced topic and are covered in a later guide called "Rails on Rack":http://edgeguides.rubyonrails.org/rails_on_rack.html.
 
 h5. Action Mailer