Update README.md
add link back to article: Hidden Classes in Java 15
@@ -4,4 +4,5 @@ This module contains articles about Java 15.
### Relevant articles
+- [Hidden Classes in Java 15](https://www.baeldung.com/java-hidden-classes)
- [Sealed Classes and Interfaces in Java 15](https://www.baeldung.com/java-sealed-classes-interfaces)
