Add CONTRIBUTING

CONTRIBUTING.md

+## Contributor guide
+
+The SOAFEE project is open for external contributors and welcomes contributions
+licensed under the [MIT license](https://spdx.org/licenses/MIT.html) and all
+accepted contributions must have the same license.
+
+Any documentation contributions are to be made under the [Creative Commons
+Attribution 4.0 International License](https://spdx.org/licenses/CC-BY-4.0.html)
+and all accepted contributions must have the same license.
+
+## Developer Certificate of Origin (DCO)
+
+Before the SOAFEE project accepts your contribution, you need to certify its
+origin and give us your permission. To manage this process we use a [Developer
+Certificate of Origin (DCO) V1.1](https://developercertificate.org/).
+
+To indicate that you agree to the the terms of the DCO, you "sign off" your
+contribution by adding a line with your name and e-mail address to every git
+commit message:
+
+````bash
+Signed-off-by: John Doe john.doe@example.org
+````
+
+You must use your real name, no pseudonyms or anonymous contributions are
+accepted.
+
+## Releases
+
+Official SOAFEE releases are published through the official SOAFEE GitLab
+repository.
+
+## Development repository
+
+The SOAFEE development repository is hosted at https://gitlab.arm.com/soafee by
+Arm.
+
+## Communications
+
+We encourage developers who are interested in SOAFEE to communicate through the
+gitlab instance provided, or other community agreed mechanisms.