Add a list of time-stamp authorities

[crandmck]

Add a list of time-stamp authorities for user reference to to the docs.

Caveat that this is not an endorsement as with CAs.

[crandmck]

The C2PA spec specifies RFC 3161 timestamps but with some additional requirements. What does the user need to do to satisfy those requirements?

Add a list of time-stamp authorities for user reference. Caveat that this is not an endorsement as with CAs.

For example: https://gist.github.com/Manouchehri/fd754e402d98430243455713efada710

[crandmck]

Update when we have the public C2PA Trust List.