Update README.md

[skip ci]

Author: edizor

kotlin-testing/README.md

@@ -9,3 +9,4 @@ This module contains articles about testing in Kotlin
 - [MockK: A Mocking Library for Kotlin](https://www.baeldung.com/kotlin/mockk)
 - [Testing a Lambda Function With Mockito Kotlin](https://www.baeldung.com/kotlin/mockito-verify-lambda)
 - [How to Mock and Verify Lambda Expression in Kotlin](https://www.baeldung.com/kotlin/lambda-mock-verify)
+- [Testing Extension Methods in Kotlin](https://www.baeldung.com/kotlin/test-extension-methods)