[Release 2.1] Plan the release

[trumant]

As discussed in the Orbit WG meeting today - @eddie-knight and @jmeridth and @trumant to discuss how to cut a v2.1.0 release of the specification and what that means in terms of releasing tooling in si-tooling

[trumant]

We should decide if #134 and ossf/si-tooling#26 should make the release or not.

I believe the documentation within the schema and the resulting improvements in documentation to si-tooling provide enough contributor and maintainer benefit to warrant inclusion.

[trumant]

See also #106 which is the primary motivation for this release.