Add content to OVSDB Southbound plugin developers manual

Add content to OVSDB Southbound plugin developers manual

Adds information to the OVSDB Southbound MD-SAL developer
manual.  Current build of OVSDB southbound was not stable,
so more examples will need to added once things stabilize.

v2 - fixup review comments
v3 - add description about how the the opendaylight-iid
     entry in the open_vswitch external-ids column is
     used to control the node-id of the OVSDB node in
     the MD-SAL.
v4 - Openvswitch -> Open vSwitch

Change-Id: Ie0f587133a37df04fdaa3dd3cc422f32e3a40136
Signed-off-by: Eric Multanen <eric.w.multanen@intel.com>