Provide info how the known_hash can be discovered?

[erikvansebille]

Provide info how the known_hash can be discovered?

Originally posted by @erikvansebille in #25 (comment)

[fluidnumerics-joe]

I'll bring this back while I'm adding the data for ICON.
The workflow I'm thinking for this is that someone could provide a url for publicly accessible data and we use pooch to download it and create the hash.

What I might do in the process is move the data registry into a json file that can be stored at the root of this repository to make it more apparent where the registry is defined. This will also allow us to use the discovery process to update the registry on the fly.