Update README.md

Author: johnA1331

core-scala/README.md

@@ -11,3 +11,4 @@ This module contains articles about Scala's core features
 - [Tail Recursion in Scala](https://www.baeldung.com/scala/tail-recursion)
 - [Guide to lazy val in Scala](https://www.baeldung.com/scala/lazy-val)
 - [Partial Functions in Scala](https://www.baeldung.com/scala/partial-functions)
+- [Introduction to Testing With ScalaTest](https://www.baeldung.com/scala/scalatest)