Add compliance note for Developer Certificate of Origin

Added a note about compliance with the Developer Certificate of Origin.

Author: adrolli

CONTRIBUTING.md

@@ -51,5 +51,7 @@ If the project maintainer has any additional requirements, you will find them li
 - **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.
 
 - **Send coherent history** - Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash them](http://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
+  
+- Before adding code, please make sure that your contribution is compliant to the [Developer Certifcate of Origin](https://developercertificate.org).
 
 **Happy coding**!