Add documentation for the mount method in ActionDispatch's Mapper

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -262,6 +262,23 @@ def match(path, options=nil)
           self
         end
 
+        # Mount a Rack-based application to be used within the application.
+        #
+        # mount SomeRackApp, :at => "some_route"
+        #
+        # Alternatively:
+        #
+        # mount(SomeRackApp => "some_route")
+        #
+        # All mounted applications come with routing helpers to access them.
+        # These are named after the class specified, so for the above example
+        # the helper is either +some_rack_app_path+ or +some_rack_app_url+.
+        # To customize this helper's name, use the +:as+ option:
+        #
+        # mount(SomeRackApp => "some_route", :as => "exciting")
+        #
+        # This will generate the +exciting_path+ and +exciting_url+ helpers
+        # which can be used to navigate to this mounted app.
         def mount(app, options = nil)
           if options
             path = options.delete(:at)