Add warning about IE[67] to fullscreen demo

Author: marijnh

demo/fullscreen.html

@@ -150,5 +150,9 @@ <h1>CodeMirror 2: Full Screen Editing</h1>
 
     <p>Press <strong>F11</strong> (or <strong>ESC</strong> in Safari on Mac OS X) when cursor is in the editor to toggle full screen editing.</p>
 
+    <p><strong>Note:</strong> Does not currently work correctly in IE
+    6 and 7, where setting the height of something
+    to <code>100%</code> doesn't make it full-screen.</p>
+
   </body>
 </html>