Adds CONTRIBUTING.md (#21)

CONTRIBUTING.md

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+# Contributing
+
+When contributing to this repository, it's recommended to first discuss the change you wish to make via issue, email,
+or any other method with the owners of this repository before making a change. However, if the change is simple enough,
+this process may be skipped and a PR may be opened directly.
+
+## Pull Request Process
+
+1. Ensure that your code is tested and follows the project's coding style.
+2. Ensure that your Pull Request follows the template defined in `.github/pull_request_template.md`.
+3. You may merge the Pull Request in once you have the sign-off of at least another developer, or if you do not have
+   permission to do that, you may request the reviewer to merge it for you.
+
+## Issuing new releases
+
+This project uses [Semver](https://semver.org/) together with Git tags in the "master" branch. Each time a tag is
+created in the "master" branch a new release will be issued, with the associated Docker images pushed to Docker Hub.
+You can see in detail how the release management is done in README.