docs: add DCO requirement to contribution guidelines (#10333)

This adds the requirement to have a DCO sign-off on commits.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

CONTRIBUTING.md

@@ -47,6 +47,53 @@ Updates to the [documentation site](https://docs.syncthing.net/) can be
 made as pull requests on the [documentation
 repository](https://github.com/syncthing/docs).
 
+## The Developer Certificate of Origin (DCO)
+
+The Syncthing project requires the Developer Certificate of Origin (DCO)
+sign-off on pull requests (PRs). This means that all commit messages must
+contain a signature line to indicate that the developer accepts the DCO.
+
+The DCO is a lightweight way for contributors to certify that they wrote (or
+otherwise have the right to submit) the code and changes they are
+contributing to the project. Here is the full [text of the
+DCO](https://developercertificate.org):
+
+---
+
+By making a contribution to this project, I certify that:
+
+1. 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
+
+2. 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
+
+3. The contribution was provided directly to me by some other person who
+   certified (1), (2) or (3) and I have not modified it.
+
+4. 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.
+
+---
+
+Contributors indicate that they adhere to these requirements by adding
+a `Signed-off-by` line to their commit messages.  For example:
+
+    This is my commit message
+
+    Signed-off-by: Random J Developer <random@developer.example.org>
+
+The name and email address in this line must match those of the committing
+author.
+
 ## Licensing
 
 All contributions are made available under the same license as the already