Adding free object-oriented programming course (#6661)

Adding free object-oriented programming course on YouTube by Kunal Kushwaha

courses/free-courses-en.md

@@ -499,6 +499,7 @@
 * [Java Tutorial for Complete Beginners](https://www.udemy.com/course/java-tutorial/) - John Purcell (Udemy)
 * [Learn how to program: Java](https://www.learnhowtoprogram.com/java-june-2017) - Epicodus Inc.
 * [Learn Java](https://www.sololearn.com/learning/1068) - Sololearn *(email address required)*
+* [Object Oriented Programming (OOP) In Java Course](https://www.youtube.com/playlist?list=PL9gnSGHSqcno1G3XjUbwzXHL8_EttOuKk) - Kunal Kushwaha (YouTube)
 * [Object-Oriented programming with Java, part I](https://moocfi.github.io/courses/2013/programming-part-1/)
 * [Object-Oriented programming with Java, part II](https://moocfi.github.io/courses/2013/programming-part-2/)
 * [Princeton Algorithms, Part 1](https://www.coursera.org/course/algs4partI)