RFR:8166632: Document how to grant permissions for a module jrt:/ in the image

[Sean Mullan]

The "jrt" URL scheme syntax is new and not widely known yet, so an 
example has been added to the conf/security/java.policy file to help 
users show how to grant permissions to a module:

http://cr.openjdk.java.net/~mullan/webrevs/8166632/webrev.00/
Thanks,
Sean