Registering custom validation methods in a module

Thx for the reply. I know this way from the docs. However I want my module to be self-contained. The only change in app/Config/* should be enabling the module through adding its namespace. Everything else, including configuration extensions related to the module should be handled by the module. I have a feeling that this is achievable, just can't get Contact/Config/Registrar.php and Validation() static method get to update Validation config in the app/Config/Validation before getting new instance of Validation class. The implicit registrar example in the docs is good, but somehow does not work for Validation. What am I doing wrong? Any other ideas how to add validation methods in a module without touching app/Config/* files?