[Changes made] Add MySQL for Developers course (#10964)

* Update free-courses-en.md

* Update courses/free-courses-en.md

Co-authored-by: Eric Hellman <eric@hellman.net>

---------

Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-en.md

@@ -567,6 +567,7 @@
 * [Learn SQL](https://popsql.com/learn-sql) - PopSQL
 * [Learn SQL](https://www.sololearn.com/learning/1060) - *registration required*
 * [Learn SQL: SQL Tutorial for Beginners](https://www.programiz.com/sql) - Programiz
+* [MySQL for Developers](https://planetscale.com/learn/courses/mysql-for-developers/introduction/course-introduction) - Aaron Francis (PlanetScale)
 * [MySQL Playlist](https://www.youtube.com/playlist?list=PLZoTAELRMXVNMRWlVf0bDDSxNEn38u9Cl) - Krish Naik
 * [MySQL Tutorial for Beginners [Full Course]](https://www.youtube.com/watch?v=7S_tz1z_5bA) - Moshfegh Hamedani (Programming with Mosh)
 * [SQL Foundations](https://www.udemy.com/course/sql-essentials-for-beginners/) - Sagar Uppuluri (Udemy)