Document the additional configuration requirement for Jetty 8+

Since Jetty 8, the org.eclipse.jetty.servlet.ServletContextHandler.Decorator system class is not visible from the webapp. It's necessary to make this class visible so that the Weld is able to decorate some Jetty internal classes.

See also http://www.eclipse.org/jetty/documentation/current/jetty-classloading.html#setting-server-classes.

For the record: it seems that the error does not occur when running in embedded mode.