docs: Release notes should be updated in a separate commit

Updating docs/news.xml in the same commit that performs the
documented change makes backports needlessly complicated,
both for mainteinance branches and downstream distributions,
because it introduces additional potential for merge
conflicts.

Document in the contributor guidelines that the release notes
should be updated in a separate commit instead, so that it's
easy to backport just the code change.

docs/hacking.html.in

@@ -302,7 +302,8 @@
         or fixing all but the most obscure bugs, must be (briefly) described
         in a release notes entry; changes that are only relevant to other
         libvirt developers, such as code refactoring, don't belong in the
-        release notes.</p>
+        release notes. Note that <code>docs/news.xml</code> should be updated
+        in its own commit not to get in the way of backports.</p>
       </li>
     </ol>