chore: End-to-end tests are mandatory for newscalers (#1808)

Signed-off-by: Tom Kerkhove <[email protected]>

CONTRIBUTING.md

@@ -41,7 +41,7 @@ Information on how scalers work can be found in [`CREATE-NEW-SCALER`](CREATE-NEW
 
 ### Testing
 
-It is highly recommended adding scaler together with end to end test (e2e). For more information on e2e testing in KEDA
+It is mandatory to provide end-to-end (e2e) tests for new scaler. For more information on e2e testing in KEDA
 check the [test documentation](./tests/README.md). Those tests are run nightly on our
 [CI system](https://github.com/kedacore/keda/actions?query=workflow%3A%22nightly+e2e+test%22).