Merge pull request EbookFoundation#1501 from maddin77/patch-1

Update free-programming-books.md

Author: vhf

free-programming-books.md

@@ -1169,6 +1169,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [Dev Docs](http://devdocs.io/)
 * [Developing Backbone.js Applications](http://addyosmani.github.io/backbone-fundamentals/) - Addy Osmani
 * [Eloquent JavaScript 2nd edition](http://eloquentjavascript.net/) - Marijn Haverbeke
+* [Exploring ES6](https://leanpub.com/exploring-es6/read) - Dr. Axel Rauschmayer
 * [Google JavaScript Style Guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml)
 * [Human Javascript](http://read.humanjavascript.com/)
 * [JavaScript Allongé](https://leanpub.com/javascript-allonge/read) - Reginald Braithwaite