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]: #507 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

image-layout.md

@@ -69,6 +69,8 @@ Those tags will often be represented in an image-layout repository with matching
 
 ### Example Ref
 
+This is an example `v1.0` ref with a manifest-list descriptor:
+
 ```
 $ cat ./refs/v1.0 | jq
 {