news: Document more possible sections

The "Security" section has been used in the past, so we're only
documenting existing behavior; the "Packaging changes" will be
used in the next commit, as well as in future releases when we
make more changes that are relevant to packagers, such as the
switch to Meson.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NLaine Stump <laine@redhat.com>

docs/news.xml

@@ -30,10 +30,13 @@
     </section>
   </release>
 
-    If relevant for a given release, add a section for removed
-    features too
+    If relevant for a given release, add a section for these too:
 
+    <section title="Packaging changes">
+    </section>
     <section title="Removed features">
+    </section>
+    <section title="Security">
     </section>
 
      -->