Added comment for spring-servlet.xml

openid-connect-server/src/main/webapp/WEB-INF/spring-servlet.xml

@@ -3,5 +3,8 @@
 	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
 
+<!-- This file has been left blank -->
+<!-- Feel free to override this by using a maven overlay. -->
+<!-- see: https://github.com/mitreid-connect/OpenID-Connect-Java-Spring-Server/wiki/Maven-Overlay-Project-How-To/ -->
 
 </beans>