Add a free JS resource on the list of free JS books. (#3260)

* Add a free JS resource on the list of free JS books.

* Alphabetical orderFix

"let's" > "leaflet"

* orderFix with eyes wide open.

T > A.

free-programming-books.md

@@ -1508,6 +1508,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Leaflet Tips and Tricks: Interactive Maps Made Easy](https://leanpub.com/leaflet-tips-and-tricks/read) - Malcolm Maclean
 * [Learn to Code JavaScript by Playing a Game](http://codecombat.com)
 * [Learning JavaScript Design Patterns](http://addyosmani.com/resources/essentialjsdesignpatterns/book/) - Addy Osmani
+* [Let's Learn ES6](https://bubblin.io/cover/let-s-learn-es6-by-ryan-christiani#frontmatter) - Ryan Christiani (Superbook format)
 * [Managing Space and Time with JavaScript - Book 1: The Basics](http://www.noelrappin.com) - Noel Rappin
 * [Marionette Exposé: Learn to write modular Javascript applications using Backbone Marionette and RequireJS/AMD](https://leanpub.com/marionetteexpose) - Jack Killilea  *(Leanpub account or valid email requested)*
 * [Mastering JavaScript High Performance](https://www.packtpub.com/packt/free-ebook/javascript-high-performance) - Chad R. Adams, Packt. (email address *requested*, not required)