Official Proposal: json-api/json-api#1254

Changes required after publishing the official proposal:

• Remove extensions requirement since it has been moved to the base spec. Consider taking the
  extension work and moving it to a new branch off master to support the extensions proposal (ref ?).
• Remove json pointers implementation, this is no longer part of the spec. This work should be preserved in some form (either by not squashing after removal or moved to a different branch).
• Add support for local ids Introduce Local IDs to v1.1 json-api/json-api#1244
• ...