nixos/release-notes for 18.09: fix missing entry

- the addition of the groups kvm and render breaks the configuration of
   users, which added them

nixos/doc/manual/release-notes/rl-1809.xml

@@ -637,6 +637,11 @@ $ nix-instantiate -E '(import <nixpkgsunstable> {}).gitFull'
      anyways for clarity.
     </para>
    </listitem>
+   <listitem>
+    <para>
+        Groups <literal>kvm</literal> and <literal>render</literal> are introduced now, as systemd requires them.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>