[contributor] add a note about the preferred image format
[contributor] add a note about the preferred image format

Change-Id: I127e1cb31f7258be5a3fac21ed61631665cf7f50

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals