quickcheck-state-machine-0.9.0: CONTRIBUTING.md

## Contributing

Contributions are most welcome!

Please open an issue before embarking on a big change. Before you open a PR,
try to:

  1. Update CHANGELOG.md;
  2. Add `@since $VERSION`, if appropriate;
  3. Update CI in `.github/workflows/main.yaml` if, for example, new GHC support
     is added.