Added Next.js and "Mastering Next.js" course (#4497)

* Added Next.js and "Mastering Next.js" course

* spaces

* no trailing slash

free-courses-en.md

@@ -26,6 +26,7 @@
 * [JavaScript](#javascript)
   * [Angular.js](#angularjs)
   * [jQuery](#jquery)
+  * [Next.js](#nextjs)
   * [NodeJS](#nodejs)
   * [React](#react)
   * [React Native](#react-native)
@@ -311,6 +312,11 @@
 * [Introduction to JQuery](https://www.udacity.com/course/intro-to-jquery--ud245) (Udacity)
 
 
+#### Next.js
+
+* [Mastering Next.js](https://masteringnextjs.com)
+
+
 #### NodeJS
 
 * [Beginner's Series to: Node.js](https://youtube.com/playlist?list=PLlrxD0HtieHje-_287YJKhY8tDeSItwtg) - Microsoft Developer (YouTube)