layer: clarify that not all file types need be supported
layer: clarify that not all file types need be supported

Signed-off-by: John Starks <4f08c47f7080d055a9777fa88698f74c617e54d1@microsoft.com>

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