Update CONTRIBUTING.md

Signed-off-by: Jan Kowalleck <[email protected]>

Author: jkowalleck

CONTRIBUTING.md

@@ -7,8 +7,8 @@ first.
 
 ## Sign off your commits
 
-Please sign off your commits, 
-to show that you agree to publish your changes under the current terms and licenses of the project.
+Please sign off your commits, to show that you agree to publish your changes under the current terms and licenses of the project
+, and to indicate agreement with [Developer Certificate of Origin (DCO)](https://developercertificate.org/).
 
 ```shell
 git commit --signed-off ...