add CONTRIBUTING file

CONTRIBUTING.md

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+# Contributing to edutap.wallet_google
+
+Be excellent to each other!
+
+All contributions are under the [EUPL 1.2](https://eupl.eu/).
+
+Do not use material protected by copyright, patents or similar, only your very own work.
+
+## We love contributions
+
+When contributing to this repository, please first discuss the change you wish to make via issue, with the core contributors of this repository before making a change.
+
+We encourage you to contribute to this project.
+Contributing is possible by i.e.:
+- spreading the word about this package
+- reporting issues
+- proposing changes/improvements to the documentation
+- fixing issues
+- improving tests
+- proposing/discussing features
+- implementing features
+
+## Reporting issues
+
+Issues are reported at the [packages issue tracker at Github](https://github.com/edutap-eu/edutap.wallet_google/issues).
+
+Please search for existing issues on GitHub before reporting new ones.
+This approach minimizes duplicates, streamlines issue management, and saves time for both reporters and maintainers.
+
+Use the issue template addressing your problem.
+
+## Pull Requests
+
+First check if someone else already created a pull request solving your problem.
+If so, get in contact with the author(s) and coordinate with them.
+
+If you're the first, fork the repository and work on the code.
+We recommend to create a branch in the fork too in order to make it easier to update the base branch.
+
+Create a draft pull request after the first commit and mark it as draft.
+This way you communicate your work to other developers, reducing the risk of duplicate work,
+
+While working on the pull request ensure to run tests and linter before each commit and push.
+
+If work is ready, remove the draft status from the PR to signalize readiness for review.
+
+On of the core contributors will review, comment and - if all is fine - merge it.
+
+## Documentation
+
+We love improvements to the documentation.
+
+We try to follow the [Diátaxis](https://diataxis.fr/) approach as best as we can.
+
+The documentation for this package is written to be rendered with Sphinx and MyST (a Markdown dialect).
+The actual documentation is located at our [documentation](https://github.com/edutap-eu/documentation/) repository.
+This package is there included as Git submodule.
+However, do not edit in a clone of the documentation repository, but here.
+
+Follow the pull request path also for documentation changes.