add cs50's game development course (#7715)

courses/free-courses-en.md

@@ -462,6 +462,7 @@
 * [[Beginner] Make an RPG](https://www.youtube.com/playlist?list=PL9FzW-m48fn2ug_FSNnfozQs3qYlBNyTd) - HeartBeast
 * [2D Hack-n-Slash Course (Complete Course) - GameMaker Studio 2](https://www.youtube.com/playlist?list=PL9FzW-m48fn0mblTG_KFDg81AMXDPKBE5) - HeartBeast
 * [CS50 2019 - Games Track](https://www.youtube.com/playlist?list=PLhQjrBD2T382mHvZB-hSYWvoLzYQzT_Pb) - David J. Malan (Harvard OpenCourseWare)
+* [CS50’s Introduction to Game Development](https://cs50.harvard.edu/games/) - Colton Ogden (Harvard OpenCourseWare and edX)
 * [Game Design and Development 1: 2D Shooter (Unity)](https://coursera.org/share/20c8b45e02110782aff633467f979c81) - Brian Winn (Coursera)
 * [GameMaker Studio 2](https://www.youtube.com/playlist?list=PL9FzW-m48fn1CFiBHB1liGKIyVO9j3A-I) - HeartBeast
 * [GameMaker Studio 2 - Action RPG Tutorial](https://www.youtube.com/playlist?list=PLPRT_JORnIuosvhfax2TQTEmN7OYTcSvK) - Shaun Spalding