+ add comment about Tomcat 6

src/main/webapp/META-INF/context.xml

 <!-- Tomcat context descriptor used for specifying a custom ClassLoader -->
 <!-- Useful for allowing web-app wide (as opposed to VM or container wide) class instrumentation -->
 <Context path="/petclinic" reloadable="false">
-	<!-- please note that useSystemClassLoaderAsParent is available since Tomcat 5.5.20 / remove if previous versions are being used -->
+	<!-- please note that useSystemClassLoaderAsParent is available only in Tomcat 5.5.x since Tomcat 5.5.20. Remove it if Tomcat 6 is used -->
 	<!--  
 	<Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" useSystemClassLoaderAsParent="false"/>
 	-->