Add Bart van Riel's tutorial.

doc/xdocs/articles.html

@@ -40,6 +40,9 @@
     <li><b><a href="http://www.javalobby.org/articles/acegisecurity/part1.jsp">Securing Your Java Applications - Acegi Security Style</a></b>:
 		Matthew Porter wrote this good introductory article for Javalobby.<br><br>
 	</li>
+    <li><b><a href="http://home.hccnet.nl/bart.van.riel/">Acegi Spring Tutorial</a></b>:
+		Available in PDF and HTML formats, thanks to Bart van Riel.<br><br>
+	</li>
     <li><b><a href="http://jroller.com/page/paskos?entry=acegi_portable_independent_and_rich">Acegi: Portable, Independent and Rich Webapp Security</a></b>:
 		Pascal Gehl relates his experience in migrating from CMA to Acegi Security.<br><br>
 	</li>