Document how DID Resolvers can attest to the resolution results they provide

[wip-abramson]

We should document how DID Resolvers can attach proofs to DID resolution results in resolution metadata that attest to the resolution result they are providing.

Also, we probably want a resolution option that specifies if you want a signed attestation from the DID Resolver for the request being made.

This is also important when DID Resolvers proxy to other Resolvers see #159

[wip-abramson]

This could also enable secure DID Resolvers without HTTPS, see #93