Add  note on how to generate the reference guide and where its source is kept at head of the doc

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1481167 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-311-trunk-gd