add missing CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+## Security
+
+If you think you have found a **security issue**, please do not mention it in this repository.
+Instead, send an email to security@thegeeklab.de with as many details as possible so it can be handled confidential.
+
+## Bug Reports and Feature Requests
+
+If you have found a **bug** or have a **feature request** please use the search first in case a similar issue already exists.
+If not, please create an issue in this repository
+
+## Code
+
+If you would like to fix a bug or implement a feature, please fork the repository and create a Pull Request.
+
+Before you start any Pull Request, it is recommended that you create an issue to discuss first if you have any
+doubts about requirement or implementation. That way you can be sure that the maintainer(s) agree on what to change and how,
+and you can hopefully get a quick merge afterwards.
+
+Pull Requests can only be merged once all status checks are green.
+
+## Do not force push to your Pull Request branch
+
+Please do not force push to your Pull Requests branch after you have created your Pull Request, as doing so makes it harder for us to review your work.
+Pull Requests will always be squashed by us when we merge your work. Commit as many times as you need in your Pull Request branch.
+
+## Re-requesting a review
+
+Please do not ping your reviewer(s) by mentioning them in a new comment. Instead, use the re-request review functionality.
+Read more about this in the [GitHub docs, Re-requesting a review](https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request#re-requesting-a-review).