Clarify license and citation guidance for Hugging Face

[egrace479]

This update to Hugging Face repo guidance includes clarifying that

• there is no license file to include, but there are specific codes to be included in the yaml of the dataset/model cards
• datasets have different license types from models and software (including update in the template)
  • Choose A License was kept in the dataset card templates since they do include a "My project isn’t software." link.
• the digital product policy should be referenced for appropriate licensing

Additionally, pointers to the data and model card templates were added on the template pages.

Any changes to gitignore guidance should likely be included as part of #44 (see note).

This PR closes #46.

[hlapp]

Looks fine. My only comment is to possibly using a more accurate term for required in the statement "Unlike in GitHub, a LICENSE.md file is not required". I think it's not only not required, but even if one is there, the UI and API won't use it to extract license information. So perhaps something more like "Unlike in GitHub, a LICENSE.md file is not required, and even if present will not be used by HF to determine license information." Or use not supported instead of not required?

[hlapp]

And BTW I would leave the markdown lint complaints alone, and move any edits you want to make to appease it into their own PR.

[egrace479]

@hlapp, that's a good idea for clarification on HF not reading the file either. In terms of the linting, the templates weren't meant to be run through (it's a bug, being fixed in #53).