Update comment to make CHAINED model reference extraction requirements clear

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/trunk@1611847 13f79535-47bb-0310-9956-ffa450edef68
Add comments about versioning

git-svn-id: https://svn.apache.org/repos/asf/manifoldcf/trunk@1611848 13f79535-47bb-0310-9956-ffa450edef68