Backwards Compatibility and Versioning

[GarethCOliver]

This issue is tracking formalizing the approach to backwards compatibility and versioning.

This includes:

• Defining what is a 'breaking' changing:
  • Essentially which versions will still work with each other without changes
• Setting the expectations for implementers as to the stability of the spec and when and how breaking changes will be allowed
  • Something like:
    • Draft / Implementors Draft: Expect breaking changes until final
    • Major Version: Allows breaking changes, but prefers not to
    • Minor Version: Must not introduce breaking changes in existing features, but may introduce new features

Additionally we should define how we intend to introduce new features and versions in-protocol:

This includes:

• High level versioning scheme
• Negotiated support for extensions / capabilities
• How to address unknown extensions / capabilities