Add note about mode dependency and links to highlighting tests.

mode/gfm/index.html

@@ -62,5 +62,9 @@ See http://github.github.com/github-flavored-markdown/.
       });
     </script>
 
+    <p>Optionally depends on other modes for properly highlighted code blocks.</p>
+
+    <p><strong>Parsing/Highlighting Tests:</strong> <a href="../../test/index.html#gfm_*">normal</a>,  <a href="../../test/index.html#verbose,gfm_*">verbose</a>.</p>
+
   </body>
 </html>