Document set-up for 3.x Hadoop based installation

[pjeli]

After having just experimented with a 3.x based cluster I have found that it may be required to run with nna.support.bootstrap.overrides=false if you are running security (Kerberos) and that you should provide a custom dfs.ha.namenode.id to satisfy the bootstrap process.

Ideally these challenges should go away when NNA becomes a simple ServicePlugin but until then we should document this so others can have an easier time with the current RPM installation.