VELOCITY-639 update 'Upgrading...' section of README for 1.6
VELOCITY-639 update 'Upgrading...' section of README for 1.6

git-svn-id: efce15992eafbd71412d600875d997890b9afd93@718342 13f79535-47bb-0310-9956-ffa450edef68

apache/velocity-engine,apache/velocity-engine