Add CONTRIBUTING.md

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

Author: jkroepke

CONTRIBUTING.md

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+# Contributing
+
+This project uses GitHub to manage reviews of pull requests.
+
+- If you are a new contributor see: [Steps to Contribute](#steps-to-contribute)
+
+- If you have a trivial fix or improvement, go ahead and create a pull request,
+  addressing (with `@...`) a suitable maintainer of this repository in the
+  description of the pull request.
+
+- If you plan to do something more involved, first discuss your ideas as
+  [GitHub issue](https://github.com/jkroepke/setup-stackit-cli/issues). This will avoid
+  unnecessary work and surely give you and us a good deal of inspirations.
+
+- Be sure to sign off on the [DCO](https://github.com/probot/dco#how-it-works).
+
+## Steps to Contribute
+
+Should you wish to work on an issue, please claim it first by commenting on the
+GitHub issue that you want to work on it. This is to prevent duplicated efforts
+from contributors on the same issue. For quickly compiling and testing your
+changes do:
+
+```bash
+# For building.
+npm run package
+
+# For testing.
+npm run all         # Make sure all the tests pass before you commit and push :)
+```
+
+## Pull Request Checklist
+
+- Branch from the main branch and, if needed, rebase to the current main branch
+  before submitting your pull request. If it doesn't merge cleanly with main you
+  may be asked to rebase your changes.
+
+- Commits should be as small as possible, while ensuring that each commit is
+  correct independently (i.e., each commit should compile and pass tests).
+
+- The PR title should be of the format: `prefix: what this PR does` (for
+  example, `feat: Add support for thing` or `fix: fix typo`). Valid prefixes are
+  `feat`, `fix`, `docs` and `chore`. The prefix should be followed by a colon
+  and a space.
+
+- If your patch is not getting reviewed or you need a specific person to review
+  it, you can @-reply a reviewer asking for a review in the pull request or a
+  comment.
+
+- Add tests relevant to the fixed bug or new feature.
+
+## Dependency management
+
+This project uses npm to manage dependencies on external packages.