update README.md with better glossary
update README.md with better glossary

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

mtrmac/skopeo,estesp/manifest-tool,runcom/skopeo,masters-of-cats/image,mtrmac/skopeo,mikebrow/skopeo,projectatomic/skopeo,runcom/skopeo,containers/image,jwhonce/skopeo,projectatomic/skopeo,mikebrow/skopeo,mtrmac/skopeo,jwhonce/skopeo,estesp/manifest-tool,mathewkoshy493/manifest-tool,containers/image,mathewkoshy493/manifest-tool