quickstart: add note about VPP RDMA interface

The instructions provided for VPP use the native
driver's RDMA interface. These instructions work with
Mellanox NICs only. Add note that clarifies this
limitation.

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I0233398c2d93003175ef75ec519be78fe22eb91f