[dockerfile mode] Add note about simplemode dependency

Closes #2960

mode/dockerfile/index.html

@@ -66,7 +66,8 @@ CMD ["npm","start"]
       });
     </script>
 
-    <p>Dockerfile syntac highlighting for CodeMirror.</p>
+    <p>Dockerfile syntac highlighting for CodeMirror. Depends on
+    the <a href="../../demo/simplemode.html">simplemode</a> addon.</p>
 
     <p><strong>MIME types defined:</strong> <code>text/x-dockerfile</code></p>
   </article>