Also mention Pattern Synonyms (but with a FIXME)

Signed-off-by: Austin Seipp <austin@well-typed.com>

docs/users_guide/7.8.1-notes.xml

@@ -83,6 +83,17 @@
            </para>
        </listitem>
 
+        <listitem>
+            <para>
+                GHC now supports pattern synonyms, enabled by the
+                <literal>-XPatternSynonym</literal> extension,
+                allowing you to name and abstract over patterns more
+                easily.
+
+                For more information, see TODO FIXME.
+           </para>
+       </listitem>
+
         <listitem>
             <para>
                 There has been significant overhaul of the type