Add issue stats to README

We should probably make it clearer how active the development is and also how quickly we close issues. Will help motivate us to do better and also clarify to everyone else that we are _really_ active.

Author: joshsmith

README.md

@@ -1,6 +1,6 @@
 # ActiveModel::Serializers
 
-[![Build Status](https://travis-ci.org/rails-api/active_model_serializers.svg)](https://travis-ci.org/rails-api/active_model_serializers)
+[![Build Status](https://travis-ci.org/rails-api/active_model_serializers.svg)](https://travis-ci.org/rails-api/active_model_serializers) [![Issue Stats](http://issuestats.com/github/rails-api/active_model_serializers/badge/pr)](http://issuestats.com/github/rails-api/active_model_serializers) [![Issue Stats](http://issuestats.com/github/rails-api/active_model_serializers/badge/issue)](http://issuestats.com/github/rails-api/active_model_serializers)
 
 ActiveModel::Serializers brings convention over configuration to your JSON generation.