Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -3,3 +3,13 @@ Contributing is always welcome!
 I am no professional flask developer, if you know a better way that something can be done, please let me know!
 
 Otherwise, it's always best to PR into the `dev` branch.
+
+Please be sure that all new functionality has a matching test!
+
+Use `pytest` to validate/test, you can run the existing tests as `pytest tests/test_notifications.py` for example
+
+```
+pip3 install -r requirements-dev
+```
+
+this is from https://github.com/dgtlmoon/changedetection.io/blob/master/requirements-dev.txt