README: Document stage3-based rootfs.tar.gz generation
README: Document stage3-based rootfs.tar.gz generation

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

liangchenye/ocitools,opencontainers/ocitools,wking/ocitools-v2,hmeng-19/ocitools,hmeng-19/ocitools,wking/ocitools-v2,opencontainers/runtime-tools,liangchenye/ocitools,opencontainers/ocitools,opencontainers/runtime-tools