Release Candidate Signature Format

[iamsamirzon]

Summary
The final release candidate signature format. This may or may not be the same as the one in Alpha release
Intended Outcome
The specfication PR is merged
Additional context
(exploring PKCS7) Need to start now on this, to define spec; and then decide the incremental approach, and when to release it;

Link to the issue in the appropriate repository: notaryproject/specifications#105

[iamsamirzon]

Finalized in NV2 meeting on 4/28 to use the existing JWS format uses for Alpha-1 and Alpha-2 to be the RC-1 format. Need to update the spec, and then close this item

[iamsamirzon]

Milind - Could you do a PR on this. Once merged, I can close this and the related roadmap item #6

[iamsamirzon]

Will close this roadmap item, when this PR is merged : notaryproject/specifications#158

[iamsamirzon]

With the above merged. I am ok with closing it. Discussed in the NV2 meeting on 6/23 with @dtzar , @iamsamirzon , @roywill