document DCO requirement

README.md

@@ -22,8 +22,8 @@ process:
 list](http://www.slf4j.org/mailing-lists.html) about your proposed
 change. Alternately, file a [bug
 report](http://www.slf4j.org/bug-reporting.html) to initiatite the
-discussion. Note that we usually ask most pull requests to be linked
-to a Jira ticket.
+discussion. Note that we ask pull requests to be linked to a [Jira
+ticket](https://jira.qos.ch/).
 
 2. Fork qos-ch/slf4j. Ideally, create a new branch from your fork for
 your contribution to make it easier to merge your changes back.
@@ -32,6 +32,9 @@ your contribution to make it easier to merge your changes back.
 sure that your code passes existing unit tests. Please add unit tests
 for your work if appropriate. It usually is.
 
+All commits must have signed off by the contributor attesting to
+[Developer Certificate of Origin (DCO)](https://developercertificate.org/).
+
 4. Push your changes to your fork/branch in github. Don't push it to
 your master! If you do it will make it harder to submit new changes
 later.