Changing requirements for new did method registry entries

[OR13]

Originally raised in: https://github.com/w3c/did-spec-registries/pull/360#discussion_r744282634

We are not sure if there should be additional required fields associated with registry entries.

Today, we require: "name", "identifier", "specification".

Tomorrow, we might require contactEmail, etc....

What fields should be required when registering?

What should their legal values be?