Release notes: mention -XEmptyCase

docs/users_guide/7.8.1-notes.xml

@@ -297,6 +297,17 @@
            </listitem>
        </itemizedlist>
 
+        <itemizedlist>
+            <listitem>
+                <para>
+                    There is a new extension,
+                    <literal>EmptyCase</literal>, which allows
+                    to write a case expression with no alternatives
+                    <literal>case ... of {}</literal>.
+               </para>
+           </listitem>
+       </itemizedlist>
+
         <itemizedlist>
             <listitem>
                 <para>