add section about DCO to CONTRIBUTING.md

Author: dirk-thomas

CONTRIBUTING.md

@@ -11,3 +11,8 @@ be under the Apache 2 License, as dictated by that
    the terms of any separate license agreement you may have executed
    with Licensor regarding such Contributions.
 ~~~
+
+Contributors must sign-off each commit by adding a `Signed-off-by: ...`
+line to commit messages to certify that they have the right to submit
+the code they are contributing to the project according to the
+[Developer Certificate of Origin (DCO)](https://developercertificate.org/).