add xerces impl to help was8 to load archiva webapp

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1447247 13f79535-47bb-0310-9956-ffa450edef68
add a comment on the reason of this dependency

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1447248 13f79535-47bb-0310-9956-ffa450edef68