Documentation Updates for Plan Proposal Worfklow

I’ve updated the documentation with several new developments that rose out of the API work for plan proposals and approvals. The main branch now contains a description of the methods available to initiate or modify a plan, as well as a description of the plan statuses available. I also updated the exchange negotiation description and drawing to reflect changes since the spring; in particular, a login payload includes the plan document as base64-encoded XML, rather than just a UUID. It’s much simpler yet simultaneously more explicit.

Head on over to https://github.com/sandpiper-schemas and take a look!