new post-milestone for VCM1: Basic Issue, Claim, Sign

Following acceptance of VCM1, we'll need to develop and post a formal
vocabulary that supports the status of the spec and test suite as it stands
at that point.  As we complete additional functional milestones, the
vocabulary will evolve and grow.  Based on a discussion with dlongley, I
created VCM1.5 to track this initial effort.

As of now, there are no issues associated w/ the effort.

Here's the link: https://github.com/w3c/vc-data-model/milestone/5

-stone

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Matt Stone
501-291-1599