Update readme for 2.3 branch

Author: BookOfGreg

README.md

@@ -6,6 +6,8 @@
 [![Code Climate](https://img.shields.io/codeclimate/github/reactjs/react-rails.svg?style=flat-square)](https://codeclimate.com/github/reactjs/react-rails)
 [![Test Coverage](https://img.shields.io/codeclimate/coverage/github/reactjs/react-rails.svg?style=flat-square)](https://codeclimate.com/github/reactjs/react-rails/coverage)
 
+You are looking at the 2.3 stable branch. This branch is for maintaining the prebundled 15.6 ReactJS with Addons. Gem version 2.4.x onwards and master will no longer have React Addons.
+
 `react-rails` makes it easy to use [React](http://facebook.github.io/react/) and [JSX](http://facebook.github.io/react/docs/jsx-in-depth.html) in your Ruby on Rails (3.2+) application. Learn more:
 
 - React's [Getting Started guide](https://facebook.github.io/react/docs/getting-started.html)