Documentation: NFS/RDMA instructions for 2.6.25-rc1

Add some instructions for using the new NFS/RDMA features.

Signed-off-by: James Lentini <jlentini@netapp.com>
Cc: Roland Dreier <rdreier@cisco.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>