Run gradle check when contributing

CONTRIBUTING.md

@@ -9,7 +9,7 @@ Please note we have a code of conduct, please follow it in all your interactions
 
 ## Pull Request Process
 
-1. Make sure all tests pass and you have a passing build for the whole project.
+1. Make sure all tests pass and you have a passing build for the whole project (run `./gradlew check`).
 2. Whether you are fixing a bug or introducing a new feature, please add the necessary tests.
    It's fine to create a test named "Issue`number`Test", where `number` is the number of the GitHub issue your test 
    is reproducing, but if you can make the extra effort to add your test coverage in an existing test group that would be great <3