Added the 'documentation' to the custom parsers

README.markdown

@@ -49,7 +49,20 @@ You can also change the parser, to remove some of the fields (on the future, you
 
 		use Rack::Analytics::RequestLogger
 
-Be sure to just use one of those, since they are mutually excludent (and `only` has a preference over `except`).
+Be sure to just use one of those, since they are mutually excludent (and `only` has a preference over `except`). You can also create custom parsers, too, you just need to feed the parser with lambdas, like that:
+
+    require 'rack/analytics'
+
+    parser = Rack::Analytics::RequestParser.new
+
+    parser << lambda { |env, data| data['port'] = env['SERVER_PORT'] }
+    parser << lambda { |env, data| data['test'] = env['rack.test'] }
+
+    Rack::Analytics.parser = parser
+
+    use Rack::Analytics::RequestLogger
+
+The env is a hash with all the parameters of the request, and the data is the fields that are saved on the MongoDB. Just be creative. You can use any kind of object, though, as long that they respond to `call` with two arguments.
 
 ## Notes on Patches/Pull Requests ##