Add Built With React (enaqx#484)

Author: jpadilla

README.md

@@ -110,6 +110,7 @@ A collection of awesome things regarding React ecosystem.
 * [React Discussion Forum](https://discuss.reactjs.org/)
 * [React StackOverflow](http://stackoverflow.com/questions/tagged/reactjs)
 * [React Twitter](https://twitter.com/reactjs)
+* [Built With React](http://builtwithreact.io/)
 
 #### React Online Playground
 * [React JSFiddle Integration with JSX](https://jsfiddle.net/reactjs/69z2wepo/)