Add a reference to safe deserlialization

Author: garydgregory

src/site/xdoc/security.xml

@@ -80,5 +80,10 @@
                </ul>
             </subsection>
         </section>
-    </body>
+    <section name="Safe Deserialization">
+      <p>
+      For information about safe deserialization, please see <a href="https://commons.apache.org/io/description.html#Safe_Deserialization">Safe Deserialization</a>.
+      </p>
+    </section>
+  </body>
 </document>