Document common values to reference standards with conformsTo

[philipashlock]

Let's provide some kind of registry of common values to use for conformsTo (#362)

It looks like there's started to be some work to do this in the EU. See this comment from the work on the European DCAT profile (which contains a broken link to this csv)

[rebeccawilliams]

👍 Here would be a good place: https://project-open-data.cio.gov/open-standards/