[chore] update README.md.

Author: diasbruno

README.md

@@ -7,6 +7,14 @@ Accessible modal dialog component for React.JS
 [![Coverage Status](https://coveralls.io/repos/github/reactjs/react-modal/badge.svg?branch=master)](https://coveralls.io/github/reactjs/react-modal?branch=master)
 ![gzip size](http://img.badgesize.io/https://unpkg.com/react-modal/dist/react-modal.min.js?compression=gzip)
 
+## React 16
+
+A initial support for React 16 is available under the branch `next`.
+
+Please, when open a new PR set the target branch `next` for `[email protected]` and `master` for `2.x`.
+
+Note that it can be unstable.
+
 ## Table of Contents
 
 * [Installation](#installation)