Document how to add DCO in a hook (#7363)

Fixes #7346

Signed-off-by: Etienne Millon <me@emillon.org>

CONTRIBUTING.md

@@ -71,6 +71,12 @@ 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`.
 
+It is possible to set up `git` so that it signs off automatically by using a
+prepare-commit-msg hook in git. See <https://stackoverflow.com/a/46536244> for
+details. As noted in the manual for `format.signOff`, note that adding the
+`Signed-off-by` trailer should be a conscious act and means that you certify
+you have the rights to submit this work under the same open source license.
+
 [dco]: http://developercertificate.org/
 [js]: https://www.janestreet.com/
 [ocl]: http://ocamllabs.io/