add release note for root-key-sentinel

doc/arm/notes.xml

@@ -71,6 +71,13 @@
 	  using (now obsolete) idnkit-1 library.
 	</para>
       </listitem>
+      <listitem>
+	<para>
+	  Add root key sentinel support which enables resolvers to test
+	  which trust anchors are configured for the root.  To disable, add
+	  'root-key-sentinel no;' to named.conf.
+	</para>
+      </listitem>
     </itemizedlist>
   </section>