docs: document requirement to provide Signed-off-by lines for DCO

Document that contributors are required to assert compliance with the
Developers Certification of Origin 1.1, by providing Signed-off-by tags
for all commit messages. The DCO is formally stating what we have long
implicitly expected of contributors in terms of their legal rights to
make the contribution. This puts the project in a stronger position
should any questions around contributions be raised going forward in the
future.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

docs/hacking.html.in

@@ -108,12 +108,19 @@
           of the bug number is useful; but also summarize the issue
           rather than making all readers follow the link.  You can use
           'git shortlog -30' to get an idea of typical summary lines.
-          Libvirt does not currently attach any meaning to
-          Signed-off-by: lines, so it is up to you if you want to
-          include or omit them in the commit message.
         </p>
       </li>
 
+      <li><p>Contributors to libvirt projects <strong>must</strong>
+          assert that they are in compliance with the
+          <a href="https://developercertificate.org/">Developer
+          Certificate of Origin 1.1</a>. This is achieved by adding
+          a "Signed-off-by" line containing the contributor's name
+          and e-mail to every commit message. The presence
+          of this line attests that the contributor has read the
+          above lined DCO and agrees with its statements.
+      </p></li>
+
       <li><p>Split large changes into a series of smaller patches,
         self-contained if possible, with an explanation of each patch
         and an explanation of how the sequence of patches fits