block/rnbd: a bit of documentation

README with description of major sysfs entries, sysfs documentation
are moved to ABI dir as Bart suggested.

Link: https://lore.kernel.org/r/20200511135131.27580-25-danil.kipnis@cloud.ionos.com
Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>