Surface example JSON format more visibly

[huertanix]

A feature request: The very helpful example JSON format in the repo should be mentioned and linked to in the README. The IETF draft link may be useful for a small group of people wanting to dig deeper into the larger standardization effort around PGP key distribution, but for anyone wanting to get started making a keylist, it would be immediately helpful to mention what format it should be in.

It may also be helpful to see a link to an example when running into a "Keylist is not in JSON format" error so that admins will know what the next step is without having to dig for it.