Update README.md

Author: rcalago

core-kotlin-modules/core-kotlin-lang-4/README.md

@@ -11,4 +11,5 @@ This module contains articles about core features in the Kotlin language.
 - [Working With Triple in Kotlin](https://www.baeldung.com/kotlin/triple-type)
 - [equals() and hashCode() Generator in Kotlin](https://www.baeldung.com/kotlin/equals-hashcode-generator)
 - [The Implicit Name Of the Single Parameter in Lambda Expressions: it](https://www.baeldung.com/kotlin/lambda-expressions-it)
+- [Private Constructors in Kotlin](https://www.baeldung.com/kotlin/private-constructors)
 - [[<-- prev]](/core-kotlin-modules/core-kotlin-lang-3)