[VC Security & Trust Document] More details on Trust Model

[Macke]

Imported from AB/Connect bitbucket: https://bitbucket.org/openid/connect/issues/2008

Original Reporter: danielfett

By Giuseppe: https://bitbucket.org/openid/connect/pull-requests/468#comment-375226671

Probably we could say something more on the meaning of the trust model.

I assume that a trust model defines the mechanisms thanks to which compliant entities MUST be authenticated to a regulatory framework, federation or trust infrastructure, if you prefer.

the trust model also defines the mechanisms by which VCs, their issuers and their holders MUST be validated and verified.

the trust model should answer the question "How can I trust this RP, or this VC, or this VCI?"