Add a “command-line usage” heading and section
Add a “command-line usage” heading and section

This is not really a new content, just emphasizing the existence of
skopeo as a starting point—and highlights `copy.Image`.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

masters-of-cats/image,containers/image,containers/image