image-layout: Add English lead-in for the ref-cat example
image-layout: Add English lead-in for the ref-cat example

I don't think we need this (see the previous commit message), but
Stephen wants some additional wording:

On Tue, Dec 20, 2016 at 07:17:11PM -0800, Stephen Day wrote [1]:
> @wking This block of json just needs some human language
> introductory text. Just say what it does, reinforcing the language
> above.

I'm not sure how that squares with the wall-of-cats under "Example
Blobs", but whatever ;).

[1]: https://github.com/opencontainers/image-spec/pull/507#issuecomment-268424332

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

vbatts/oci-image-spec,jonboulle/image-spec,vbatts/oci-image-spec,opencontainers/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,mattspencer-arm/image-spec,jonboulle/image-spec