Enhance the iser readme

Enhance the iser readme  to contain more details on the RDMA
libraries needed for building and running tgt with iser.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
add support of sheepdog backing store

Hi tgt list,
(Cc-ing sheepdog list),

This patch implements a new backing store support for sheepdog, distributed
storage system initially designed for QEMU
(https://github.com/sheepdog/sheepdog). With this patch, sheepdog can support
iSCSI protocol, so hypervisors other than QEMU/KVM and native OSes can use
sheepdog as their block devices. The new --bstype is "sheepdog".
(This patch is a refinement of tgt's sheepdog branch.)

This patch still has some todos and would have bugs, but I'd like to post it for
keeping this patch small and getting feedback in early stage.

Major todos:
 - exploiting multithread support of tgt
 - support sheep processes running on different host and using pots other than
   7000
 - support unix domain socket for communicating between sheep process
 - support reconnection for safe upgrade of sheepdog

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>