Clarify Integration Test / Unit Test dependency

Author: derjust

CONTRIBUTING.md

@@ -42,4 +42,6 @@ Testing is always important and the code coverage by unit tests should not decre
 DynamoDB comes with a lot of features. Therefore new supported features should always come with an Integration Test.
 
 That means that it should have an `...IT.java` test class that uses the local DynamoDB to demonstrate how the feature is used.
-This serves as proove that it is working (test case succeeds) and also as documentation and 'how-to'.
+This serves as proove that it is working (test case succeeds) and also as documentation and 'how-to'.
+But an Integration Test should always be present _next to_ regular Unit Tests that follow common, good practise to test a single class without requiring existence of external dependencies.
+