Image building docs updated
Image building docs updated

The note added with an example of converting an image to a differnet
format.

Change-Id: Ia2bd773f847afe35a424b05f18b79ca19fb05613

ekasitk/sahara,citrix-openstack-build/sahara,esikachev/scenario,henaras/sahara,esikachev/sahara-backup,keedio/sahara,ekasitk/sahara,bigfootproject/sahara,xme1226/sahara,citrix-openstack-build/sahara,esikachev/sahara-backup,zhujzhuo/Sahara,citrix-openstack-build/sahara,zhujzhuo/Sahara,ekasitk/sahara,egafford/sahara,zhangjunli177/sahara,xme1226/sahara,matips/iosr-2015,mapr/sahara,bigfootproject/sahara,keedio/sahara,zhujzhuo/Sahara,zhangjunli177/sahara,openstack/sahara,crobby/sahara,redhat-openstack/sahara,mapr/sahara,matips/iosr-2015,zhangjunli177/sahara,mapr/sahara,xme1226/sahara,bigfootproject/sahara,esikachev/scenario,matips/iosr-2015,crobby/sahara,redhat-openstack/sahara,henaras/sahara,keedio/sahara,tellesnobrega/sahara,esikachev/scenario,tellesnobrega/sahara,henaras/sahara,esikachev/sahara-backup,openstack/sahara,redhat-openstack/sahara,egafford/sahara,crobby/sahara