Update news.xml after the post-release

In 19581afd the configure.ac was updated. However, the
news.xml was missing adjustment - creating the new section for
the release.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/news.xml

@@ -33,6 +33,14 @@
      -->
 
 <libvirt>
+  <release version="v3.7.0" date="unreleased">
+    <section title="New features">
+    </section>
+    <section title="Improvements">
+    </section>
+    <section title="Bug fixes">
+    </section>
+  </release>
   <release version="v3.6.0" date="2017-08-02">
     <section title="New features">
       <change>