Routing guide: document lambda constraints and request argument.

guides/source/routing.textile

@@ -535,6 +535,17 @@ TwitterClone::Application.routes.draw do
 end
 </ruby>
 
+You can also specify constraints as a lambda:
+
+<ruby>
+TwitterClone::Application.routes.draw do
+  match "*path" => "blacklist#index",
+    :constraints => lambda { |request| Blacklist.retrieve_ips.include?(request.remote_ip) }
+end
+</ruby>
+
+The +matches?+ method or the lambda is passed the "+request+ object":http://guides.rubyonrails.org/action_controller_overview.html#the-request-object, which means the constraint can check +request.path_parameters+, +request.headers+ or any other property of the request.
+
 h4. Route Globbing
 
 Route globbing is a way to specify that a particular parameter should be matched to all the remaining parts of a route. For example