Document versioning scheme

JUnit uses a versioning scheme similar but not identical to Semantic Versioning. For example we allow ourselves to delete or change experimental code on minor versions. Semver makes no such provisions. We communicate these differences through API-Guardian annotations.

This is non-obvious, so we should document our scheme.

Deliverables

• A versioning.md file (in API Guardian Repo?) in the root of this project that describes our versioning system.
• Update documented versioning scheme on Tidelift.