nvmetcli: README description of discovery

Some blurbs on discovery and the connection
between using nvmetcli to set up an NVMe target and
what gets seen by an NVMe Host when accessing
the discovery controller.

Signed-off-by: Jay Freyensee <james_p_freyensee@linux.intel.com>

README

@@ -111,8 +111,23 @@ The following walks through an example using interactive mode.
 ...> cd subsystems/
 ...> create testnqn
 
-Referrals/Discovery
---------------------
+Discovery
+----------
+Each NVMe Target has a discovery controller mechanism that an NVMe
+Host can use to determine the NVM subsystems it has access too.
+nvmetcli can be used to add a new record to the discovery controller
+upon each new subsystem entry and port entry that the newly
+created subsystem entry binds too (see the '/port/' and
+'/subystem/' nvmetcli walk-through earlier in this README).  Each NVMe
+Host only gets to see the discovery entries defined in
+/subsystems/[NQN NAME]/allowed_hosts and the IP port it is connected
+to the NVMe Target.
+
+A Host can retrieve these discovery logs via the nvme-cli tool
+(https://github.com/linux-nvme/nvme-cli).
+
+Referrals
+---------
 TBD
 
 Saving and restoring the configuration