Documentation: document ublk

Add documentation for ublk subsystem. It was supposed to be documented when
merging the driver, but missing at that time.

Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
[axboe: correct MAINTAINERS addition]
Signed-off-by: Jens Axboe <axboe@kernel.dk>