Add contribution rules

CONTRIBUTIONS.md

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+# Contributions
+
+## Pull Requests
+Contributions to the Lynis project should be submitted as a pull request. The upstream
+project can be found in our [GitHub repository](https://github.com/CISOfy/lynis).
+
+By submitting a [Pull Request](https://help.github.com/articles/using-pull-requests/)
+to this repository, you agree that you:
+
+1. Own the contribution that you are providing, or have obtained permission from
+   the contribution owner
+
+2. Allow your contribution to be licensed under the license of the target
+   project (GPLv3)
+
+3. Allow your contribution to be freely distributed to the Lynis community
+
+4. Allow the project the [Unlimited Rights](#Unlimited-Rights) to your contribution
+
+
+## Unlimited Rights
+Our project is licensed under GPLv3. By providing a contribution to the project, it
+will be used for the purpose of the project. Unlimited rights includes the rights to
+use, modify, reproduce, release, perform, display, or disclose computer software or
+computer software documentation in whole or in part, in any manner and for any
+purpose whatsoever, and to have or authorize others to do so.
+
+If you want to be named in as a contributor in the CONTRIBUTOR file, then include
+this notition in your pull request (format: Full Name, with optional the company name
+and/or your e-mail address).
+