Document digest specification? about go-digest

[stevvooe]

@wking I think we can add a reference to Docker Registry HTTP API V2 and OCI Image Spec.

Encoding is a different issue. Basically, it is a per-alg detail. The addition of alt encoding schemes isn't a part of this package. In practice, this hasn't been an issue but we may want to narrow this package's validation.

Let me update this in the ecosystem before getting too deep here.

[stevvooe]

After some thought, it is best that this package supports a superset of what is valid in the OCI specification. This package will validate the minimum safe level of digest format. Anything further than that can be done in the specification specific validator.

Thank you for you submission!