Adding VTN content to the Developer's guide

This is Divya's section but gotten to the point that it builds by Colin
Dixon.

Change-Id: I5d97a7b1067a811cbf6f442a5038516917754944
Signed-off-by: Divya Shetty <dshetty@brocade.com>
Signed-off-by: Mathieu Lemay <mlemay@inocybe.com>
Signed-off-by: Colin Dixon <colin@colindixon.com>
Merge "Adding VTN content to the Developer's guide"