Extension for inclusion/exclusion of fields in fieldsets

Hey JSON:API enthusiasts!

Based on the needs of an API I’m working on, I have opened a discussion/RFC regarding wildcard queries for for fieldsets (RFC: Exluding fields with Sparse Fieldsets · Issue #1632 · json-api/json-api · GitHub), resulting in an extension for the JSON:API, named relfield: GitHub - ThorstenSuckow/relfield: Draft for JSON:API to extend on sparse fieldsets with excluded fields.

It’s purpose is to provide syntax and semantics for explicitly including/excluding fields for a requested resource.

I have incorporated the feedback from Dan and @jelhan, who actively reviewed the RFC. @jelhan provided some helpful insights, for which I’m thankful.

Further feedback for finalizing the extension is greatly appreciated!