move README.txt to README.md and update it

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1582202 13f79535-47bb-0310-9956-ffa450edef68
add a bit of caching here at least for released version

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1582203 13f79535-47bb-0310-9956-ffa450edef68