ACCUMULO-1010 ACCUMULO-1380 update upgrade section and API section in README

git-svn-id: https://svn.apache.org/repos/asf/accumulo/branches/1.5@1480408 13f79535-47bb-0310-9956-ffa450edef68
ACCUMULO-1010 ACCUMULO-1380 update upgrade section and API section in README

git-svn-id: https://svn.apache.org/repos/asf/accumulo/trunk@1480414 13f79535-47bb-0310-9956-ffa450edef68