Documentation: RDS: Document Multipath RDS (mprds)

Document the design of mprds, covering a brief description
of the motivation, data-structures and modifications to the
RDS control plane.

Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>