Add docs to serializable

lib/active_model/serializable.rb

@@ -1,4 +1,27 @@
+require 'active_support/core_ext/object/to_json'
+
 module ActiveModel
+  # Enable classes to Classes including this module to serialize themselves by implementing a serialize method and an options method.
+  #
+  # Example:
+  #
+  #     require 'active_model_serializers'
+  #
+  #     class MySerializer
+  #       include ActiveModel::Serializable
+  #
+  #       def initialize
+  #         @options = {}
+  #       end
+  #
+  #       attr_reader :options
+  #
+  #       def serialize
+  #         { a: 1 }
+  #       end
+  #     end
+  #
+  #     puts MySerializer.new.to_json
   module Serializable
     def as_json(args={})
       if root = args[:root] || options[:root]