doc/rbd: Added the iSCSI gateway overview content

Fixes: http://tracker.ceph.com/issues/20437
Signed-off-by: Aron Gunn <agunn@redhat.com>
(cherry picked from commit d022034bb3a6e1b00cf185cae0b5fa3f4b617a38)

diff --git a/doc/rbd/iscsi-overview.rst b/doc/rbd/iscsi-overview.rst
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+==================
+Ceph iSCSI Gateway
+==================
+
+The iSCSI gateway is integrating Ceph Storage with the iSCSI standard to provide
+a Highly Available (HA) iSCSI target that exports RADOS Block Device (RBD) images
+as SCSI disks. The iSCSI protocol allows clients (initiators) to send SCSI commands
+to SCSI storage devices (targets) over a TCP/IP network. This allows for heterogeneous
+clients, such as Microsoft Windows, to access the Ceph Storage cluster.
+
+Each iSCSI gateway runs the Linux IO target kernel subsystem (LIO) to provide the
+iSCSI protocol support and utilizes the Ceph’s RBD kernel module to expose RBD
+images to iSCSI clients. With Ceph’s iSCSI gateway you can effectively run a fully
+integrated block-storage infrastructure with all the features and benefits of a
+conventional Storage Area Network (SAN).
+
+.. ditaa::
+                  Cluster Network
+                 +-------------------------------------------+
+                 |             |               |             |
+             +-------+     +-------+       +-------+     +-------+
+             |       |     |       |       |       |     |       |
+             | OSD 1 |     | OSD 2 |       | OSD 3 |     | OSD N |
+             |    {s}|     |    {s}|       |    {s}|     |    {s}|
+             +-------+     +-------+       +-------+     +-------+
+                 |             |               |             |
+      +--------->|             |  +---------+  |             |<---------+
+      :          |             |  |   RBD   |  |             |          :
+      |          +----------------|  Image  |----------------+          |
+      |           Public Network  |    {d}  |                           |
+      |                           +---------+                           |
+      |                                                                 |
+      |                      +-------------------+                      |
+      |   +--------------+   |  iSCSI Initators  |   +--------------+   |
+      |   | iSCSI GW     |   |   +-----------+   |   | iSCSI GW     |   |
+      +-->|  RBD Module  |<--+   | Various   |   +-->|  RBD Module  |<--+
+          |              |   |   | Operating |   |   |              |
+          +--------------+   |   | Systems   |   |   +--------------+
+                             |   +-----------+   |
+                             +-------------------+
+
+
+.. toctree::
+  :maxdepth: 1
+
+  Requirements <iscsi-requirements>
+  Configuring the iSCSI Target <iscsi-targets>
+  Configuring the iSCSI Initiator <iscsi-initiators>
+  Monitoring the iSCSI Gateways <iscsi-monitoring>