Adds support for meta attribute

Currently, 0.10.0.pre doesn't support `meta` option in `render`. This
way, there's no way to support features such as pagination. `0.9` had
this feature in place.

This adds support for it, as well as fixes small things in README.md.

This won't support `meta` in array responses because arrays don't have
keys, obviously. Also, the response should have a `root` key, otherwise
no `meta` will be included.

In some cases, for example using JsonApi, ArraySerializer will result in
a response with a `root`. In that case, `meta` will be included.

test/action_controller/serialization_test.rb

@@ -37,6 +37,18 @@ module ActionController
           ]
           render json: array
         end
+
+        def render_array_using_implicit_serializer_and_meta
+          old_adapter = ActiveModel::Serializer.config.adapter
+
+          ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonApi
+          array = [
+            Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+          ]
+          render json: array, meta: { total: 10 }
+        ensure
+          ActiveModel::Serializer.config.adapter = old_adapter
+        end
       end
 
       tests MyController
@@ -87,6 +99,13 @@ module ActionController
 
         assert_equal expected.to_json, @response.body
       end
+
+      def test_render_array_using_implicit_serializer_and_meta
+        get :render_array_using_implicit_serializer_and_meta
+
+        assert_equal 'application/json', @response.content_type
+        assert_equal '{"profiles":[{"name":"Name 1","description":"Description 1"}],"meta":{"total":10}}', @response.body
+      end
     end
   end
 end