Note that we need Cabal-1.18 in the release notes

Thanks to George Colpitts for helping us remember this!
Signed-off-by: Austin Seipp <austin@well-typed.com>

docs/users_guide/7.8.1-notes.xml

@@ -255,6 +255,12 @@
                 extension.
            </para>
 
+            <para>
+                Note that you must be using Cabal and Cabal-install
+                1.18 for it to correctly build dynamic shared libraries
+                for you.
+           </para>
+
             <para>
                 Currently, Dynamic GHCi and
                 <literal>-dynamic-too</literal> are not supported on