Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-9/README.md

@@ -8,3 +8,4 @@
 - [Util Classes in Kotlin](https://www.baeldung.com/kotlin/utility-class)
 - [Result Class in Kotlin](https://www.baeldung.com/kotlin/result-class)
 - [Underscore Operator for Type Arguments in Kotlin](https://www.baeldung.com/kotlin/underscore-type-arguments)
+- [Using Enums With when() in Kotlin](https://www.baeldung.com/kotlin/enums-with-when)