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.

contribute/developer-certificate-of-origin

@@ -0,0 +1,35 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.