Add release note item for new Monoid Const instance

See [6f3a194accd2a1a75a3f86c1b4dca30521488110/base]

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

docs/users_guide/7.8.1-notes.xml

@@ -608,6 +608,11 @@
                </para>
            </listitem>
             <listitem>
+                <para>
+                    There is now a <literal>Monoid</literal> instance for <literal>Const</literal>
+               </para>
+           </listitem>
+             <listitem>
                 <para>
                     There is now a <literal>Data</literal> instance for <literal>Data.Version</literal>
                </para>