Update README.md

Author: rcalago

kotlin-testing/README.md

@@ -10,3 +10,4 @@ This module contains articles about testing in Kotlin
 - [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)
+- [Using Spy in MockK](https://www.baeldung.com/kotlin/mockk-spy)