Added npm course (#6174)

courses/free-courses-en.md

@@ -470,6 +470,7 @@
 * [Learn to Program in Javascript: Beginner to Pro](https://www.udemy.com/course/programming-in-javascript) - Raghavendra Dixit (Udemy)
 * [learn:query](https://learnquery.infinum.co)
 * [Namaste 🙏 JavaScript: An In-Depth JavaScript Fundamentals Course](https://www.youtube.com/playlist?list=PLlasXeu85E9cQ32gLCvAvr9vNaUccPVNP) - Akshay Saini (YouTube)
+* [npm - Mastering the Basics](https://www.udemy.com/course/npm-mastering-the-basics/) - Vishwas Gopinath (Udemy)
 * [Programming Foundations with Javascript, HTML and CSS](https://www.coursera.org/learn/duke-programming-web) - Owen Astrachan, Robert Duvall, Andrew D. Hilton, Susan H. Rodger (Coursera)
 * [Understanding RxJS](https://youtube.com/playlist?list=PL55RiY5tL51pHpagYcrN9ubNLVXF8rGVi) - Academind
 * [Vanilla JavaScript](https://www.youtube.com/playlist?list=PLillGF-RfqbbnEGy3ROiLWk7JMCuSyQtX) - Brad Traversy, Traversy Media (YouTube)