Update contibuter docs to mention that DCO signoff is mandatory (#429)

Signed-off-by: Abhilash Shetty <[email protected]>

CONTRIBUTING.md

@@ -19,3 +19,21 @@ make all
 Alternatively, you can open this repo in Gitpod and make your PR right from the browser:
 
 [![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/openebs/cstor-operators)
+
+---
+
+### Sign your work
+
+We use the Developer Certificate of Origin (DCO) as an additional safeguard for the OpenEBS project. This is a well established and widely used mechanism to assure contributors have confirmed their right to license their contribution under the project's license. Please read [developer-certificate-of-origin](./contribute/developer-certificate-of-origin).
+
+Please certify it by just adding a line to every git commit message. Any PR with Commits which does not have DCO Signoff will not be accepted:
+
+```
+  Signed-off-by: Random J Developer <[email protected]>
+```
+
+or use the command `git commit -s -m "commit message comes here"` to sign-off on your commits.
+
+Use your real name (sorry, no pseudonyms or anonymous contributions). If you set your `user.name` and `user.email` git configs, you can sign your commit automatically with `git commit -s`. You can also use git [aliases](https://git-scm.com/book/en/v2/Git-Basics-Git-Aliases) like `git config --global alias.ci 'commit -s'`. Now you can commit with `git ci` and the commit will be signed.
+
+---