Clarify design philosphy

[MarkLodato]

We should clearly document the design philosophy of DSSE so that we can more easily discuss issues like #33, #34, #39, and #42. I think it is something like:

• Avoid generic extensions (Extending DSSE to accept optional signature specific metadata #39) in order to avoid the problems of JWS (and because JWS already exists). If extensions are needed, encourage users to use JWS instead.
• Only add a field to the envelope if it cannot be practically added to the payload. When doing so, take care to avoid previous implementation mistakes in JWS and other protocols.