added nicholas c. zakas understanding ECMAScript 6

free-programming-books.md

@@ -977,6 +977,7 @@
 * [Speaking JavaScript](http://speakingjs.com/es5/) - Dr. Axel Rauschmayer
 * [The JavaScript Tutorial](http://javascript.info/)
 * [The Problem with Native JavaScript APIs](http://chimera.labs.oreilly.com/books/1234000001655) (PDF)
+* [Understanding ECMAScript 6](https://leanpub.com/understandinges6/read) *(Leanpub, by Nicholas C. Zakas)*
 * [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS)