Update README.md

Author: rcalago

core-java-modules/core-java-lang-oop-others/README.md

@@ -13,3 +13,4 @@ This module contains articles about Object Oriented Programming (OOP) in Java
 - [Statements Before super() in Java](https://www.baeldung.com/java-statements-before-super-constructor)
 - [Print the Default Value When Overriding toString() Method](https://www.baeldung.com/java-print-default-value-override-tostring)
 - [Pass a Class as a Parameter in Java](https://www.baeldung.com/java-pass-class-parameter)
+- [Declare an Enum in an Inner Class in Java](https://www.baeldung.com/java-declare-enum-inner-class)