Improve README file for dbimage-builder
Improve README file for dbimage-builder

Change-Id: I3cf447d87b2f437abd8b950f1141d0cd9b65ba3a

open-power-ref-design-toolkit/os-services,open-power-ref-design-toolkit/os-services,open-power-ref-design/os-services,open-power-ref-design/os-services,open-power-ref-design-toolkit/os-services