INT-738 Added documentation for the delayer endpoint

spring-integration-reference/src/gateway.xml

@@ -53,6 +53,16 @@
      @Gateway(requestChannel="orders")
      void placeOrder(Order order);
 
+ }]]></programlisting>
+    </para>
+    <para>
+      It is also possible to pass values to be interpreted as Message headers on the Message
+      that is created and sent to the request channel by using the @Header annotation:
+	  <programlisting language="java"><![CDATA[ public interface FileWriter {
+
+     @Gateway(requestChannel="filesOut")
+     void write(byte[] content, @Header(FileHeaders.FILENAME) String filename);
+
  }]]></programlisting>
     </para>
   </section>