Add details on the test script in integration/
Add details on the test script in integration/

Add details to the README about using the test script to validate an
image registry supports the manifest-list media type and Docker
distribution V2 APIs required for dealing with manifest lists.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@gmail.com>

estesp/manifest-tool,estesp/manifest-tool