Added example for LIFO

Author: TomHAnderson

doc/book/usage.md

@@ -107,6 +107,11 @@ before the "guest" role, the example code would print "allowed".
 >
 > When specifying multiple parents for a role, keep in mind that the last parent listed is the first
 > one searched for rules applicable to an authorization query.
+>
+> For example:
+> ```
+> $acl->addRole(new Role('top'), ['bottom', 'middle']);
+> ```
 
 ## Creating the Access Control List