add contributing file

Author: attwad

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+Thanks for contributing to this package!
+
+Before sending a PR, please make sure you checked the [python test workflow](.github/workflows/python-test.yml) and ran it locally, either using [act](https://nektosact.com) or by executing the workflow actions yourself.
+
+Please only send the PR once all tests pass and mypy is happy, thanks!