Add note regarding class file version issue

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/jcs/trunk@1604980 13f79535-47bb-0310-9956-ffa450edef68
Add note

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/jcs/trunk@1604982 13f79535-47bb-0310-9956-ffa450edef68
Ignore target

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/jcs/trunk@1604983 13f79535-47bb-0310-9956-ffa450edef68