Add info about QT (QT version 3 not supported anymore)

git-svn-id: http://root.cern.ch/svn/root/trunk@24509 27541ba8-7e3a-0410-8455-c3a389f83636

graf2d/doc/v520/index.html

@@ -205,3 +205,7 @@
 case.
 </ul>
 
+<h3>QT</h3>
+Version 3 of QT is not supported anymore. If you install ROOT with the QT option
+you must have QT version 4 already installed.
+