improve submitting changes section on contributing

Author: julioxavierr

CONTRIBUTING.md

@@ -57,6 +57,7 @@ Please open an issue with a proposal for a new feature or refactoring before sta
 - Create a new feature branch based off the `master` branch.
 - Make sure all tests pass and there are no linting errors.
 - Submit a pull request, referencing any issues it addresses.
+- If you changed external-facing types, make sure to also build the project locally and include the updated API report file etc/redux-toolkit.api.md in your pull request.
 
 Please try to keep your pull request focused in scope and avoid including unrelated commits.