Added info about Debian image
Added info about Debian image

This patch adds information on how to build a Debian
image from the official openstack-debian-images package.

Change-Id: I3485fea2efba1083bd8d427b41e08c455c1e698b

openstack/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual