Update contibuter docs to mention that DCO signoff is mandatory (#28)

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

CONTRIBUTING.md

@@ -29,7 +29,23 @@ OpenEBS is an Apache 2.0 Licensed project and all your commits should be signed
   - Commit body should include details such as why the changes are required and how the proposed changes.
   - DCO Signed.
 * If your PR is not getting reviewed or you need a specific person to review it, please reach out to the OpenEBS Contributors. See [OpenEBS Community](https://github.com/openebs/openebs/tree/HEAD/community).
+---
 
+### 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.
+
+---
 ## 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 that contributors have confirmed their right to license their contribution under the project's license. Please read [dcofile](https://github.com/openebs/openebs/blob/HEAD/contribute/developer-certificate-of-origin). If you can certify it, then just add a line to every git commit message: