Add dev and user meetup at hwx to front page

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1533845 13f79535-47bb-0310-9956-ffa450edef68
Edit of preface and first paragraphs of getting started

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1533852 13f79535-47bb-0310-9956-ffa450edef68