Update CONTRIBUTING.md

add mandatory paragraph about DCO

Author: phanak-sap

CONTRIBUTING.md

@@ -47,3 +47,9 @@
 Changes that are cosmetic in nature and do not add anything substantial to the
 stability, functionality, or testability of PyOData will generally not be
 accepted.
+
+## Developer Certificate of Origin (DCO)
+Due to legal reasons, contributors will be asked to accept a DCO before they submit the first pull request to this project. SAP uses the standard DCO text of the Linux Foundation.
+This happens in an automated fashion during the submission process: the CLA assistant tool will add a comment to the pull request. Click it to check the DCO, then accept it on the following screen. CLA assistant will save this decision for upcoming contributions.
+
+This DCO replaces the previously used CLA ("Contributor License Agreement") as well as the "Corporate Contributor License Agreement" with new terms which are well-known standards and hence easier to approve by legal departments. Contributors who had already accepted the CLA in the past may be asked once to accept the new DCO.