readme: mention the DCO

We haven't mentionned the DCO anywhere, yet we were requiring all
contributions to have a Signed-off-by line to accept it.

Add a reference to the DCO in our README's "development procedure"
section.

Signed-off-by: Simon Ser <contact@emersion.fr>

README.md

@@ -89,6 +89,12 @@ RFC, create a merge request and add the "WIP:" prefix in the title.
 
 To propose changes to existing protocols, create a GitLab merge request.
 
+Please include a `Signed-off-by` line at the end of the commit to certify
+that you wrote it or otherwise have the right to pass it on as an
+open-source patch. See the
+[Developer Certificate of Origin](https://developercertificate.org/) for
+a formal definition.
+
 ## Interface naming convention
 
 All protocols should avoid using generic namespaces or no namespaces in