Improve documentation of Rack::Attack::Request

https://github.com/kickstarter/rack-attack/issues/73

README.md

@@ -57,7 +57,7 @@ The algorithm is actually more concise in code: See [Rack::Attack.call](https://
 
 ```ruby
 def call(env)
-  req = Rack::Request.new(env)
+  req = Rack::Attack::Request.new(env)
 
   if whitelisted?(req)
     @app.call(env)
@@ -72,6 +72,10 @@ def call(env)
 end
 ```
 
+Note: `Rack::Attack::Request` is just a subclass of `Rack::Attack` so that you
+can cleanly [monkey patch helper methods] onto the
+[request object](https://github.com/kickstarter/rack-attack/blob/master/lib/rack/attack/request.rb).
+
 ## About Tracks
 
 `Rack::Attack.track` doesn't affect request processing. Tracks are an easy way to log and measure requests matching arbitrary attributes.

lib/rack/attack/request.rb

@@ -1,3 +1,16 @@
+# Rack::Attack::Request is the same as ::Rack::Request by default.
+#
+# This is a safe place to add custom helper methods to the request object
+# through monkey patching:
+#
+#   class Rack::Attack::Request < ::Rack::Request
+#     def localhost?
+#       ip == "127.0.0.1"
+#     end
+#   end
+#
+#   Rack::Attack.whitelist("localhost") {|req| req.localhost? }
+#
 module Rack
   class Attack
     class Request < ::Rack::Request