added a react book from digitalocean (#5937)

books/free-programming-books-langs.md

@@ -1224,6 +1224,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 * [Full-Stack React Projects](https://www.packtpub.com/free-ebooks/full-stack-react-projects) - Shama Hoque (Packt account *required*)
 * [Hacking with React](http://www.hackingwithreact.com)
+* [How To Code in React.js](https://www.digitalocean.com/community/books/how-to-code-in-react-js-ebook) - Joe Morgan
 * [Intro to the React Framework](http://code.tutsplus.com/tutorials/intro-to-the-react-framework--net-35660)
 * [Learning React.js: Getting Started and Concepts](https://scotch.io/tutorials/learning-react-getting-started-and-concepts)
 * [React-Bits](https://github.com/vasanthk/react-bits)