README.md: return to one-sentence-per-line format
README.md: return to one-sentence-per-line format

Signed-off-by: Jonathan Boulle <0d47477beef1d882e8812885f3dcf66c4a47c346@gmail.com>

opencontainers/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec