Add Intro to JavaScript ES6 programming (#3203)

* Add Intro to JavaScript ES6 programming

* fix lint

* fix lint

* fix introducing to es6 to playlist link

free-courses-en.md

@@ -217,6 +217,7 @@
 
 * [Bento JavaScript Learning Track](https://bento.io/topic/javascript) (Bento)
 * [Egghead.io](https://egghead.io)
+* [Intro to JavaScript ES6 programming](https://www.youtube.com/playlist?list=PL-xu4i_QDSxcoDNeh8rx5-pHCCTOg0XsI)
 * [Learn how to program: JavaScript](https://www.learnhowtoprogram.com/javascript) - Epicodus Inc.
 * [learn:query](https://learnquery.infinum.co)