RDMA/rtrs: a bit of documentation

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

Link: https://lore.kernel.org/r/20200511135131.27580-15-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>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>