Update README.md

A Guide to Atrium: Assertion Library for Kotlin added

Author: kwasibaeldung

kotlin-testing/README.md

@@ -6,3 +6,4 @@ This module contains articles about testing in Kotlin
 - [JUnit 5 for Kotlin Developers](https://www.baeldung.com/kotlin/junit-5-kotlin)
 - [Writing Specifications with Kotlin and Spek](https://www.baeldung.com/kotlin/spek)
 - [Testing Extension Methods in Kotlin](https://www.baeldung.com/kotlin/test-extension-methods)
+- [A Guide to Atrium: Assertion Library for Kotlin](https://www.baeldung.com/kotlin/atrium-assertion-library)