Add sign you work section in contributing and DCO

CONTRIBUTING.md

@@ -52,3 +52,22 @@ issue or contacting one or more of the project maintainers.
 This Code of Conduct is adapted from the [Contributor Covenant](http://contributor-covenant.org), 
 version 1.2.0, available at 
 [http://contributor-covenant.org/version/1/2/0/](http://contributor-covenant.org/version/1/2/0/)
+
+### Sign your work
+
+We use the Developer Certificate of Origin (DCO) as a additional safeguard
+for the Nextcloud 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 [contribute/developer-certificate-of-origin][dcofile].
+If you can certify it, then just add a line to every git commit message:
+
+````
+  Signed-off-by: Random J Developer <random@developer.example.org>
+````
+
+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/tr/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.