Updated README.md

added link back to the article: https://www.baeldung.com/hibernate-not-null-error

Author: Asjad-J

persistence-modules/hibernate-exceptions/README.md

@@ -5,3 +5,4 @@
 - [TransactionRequiredException Error](https://www.baeldung.com/jpa-transaction-required-exception)
 - [Hibernate’s “Object References an Unsaved Transient Instance” Error](https://www.baeldung.com/hibernate-unsaved-transient-instance-error)
 - [EntityNotFoundException in Hibernate](https://www.baeldung.com/hibernate-entitynotfoundexception)
+- [Hibernate’s “Not-Null Property References a Null or Transient Value” Error](https://www.baeldung.com/hibernate-not-null-error)