cover S/MIME as well as PGP/MIME

[dkg]

Speaking with Alexey Melnikov at IETF 106, he says that he thinks this approach should be fine with S/MIME.

At the LAMPS WG session there was agreement that we wanted:

• Test vectors of messages (including S/MIME):
• Screenshots of how these messages render in multiple clients

So the next revision of this draft should try to at least include the S/MIME examples.

[dkg]

See #25 for some progress on this.

[dkg]

draft -02 has S/MIME test vectors, so this is resolved.