docs/koji.md: add description for 'digest' in image extras
docs/koji.md: add description for 'digest' in image extras

Signed-off-by: Vadim Rutkovsky <2d57af434cefbff61c07aeb6241a135727942cb3@redhat.com>

fr34k8/atomic-reactor,vrutkovs/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,projectatomic/atomic-reactor