Spec contributor's guide

Before our next big spec updates, it would be great if someone wants to write a short guide on spec editing with a section on terminology so we use consistent terms for the runtime, converters, implementation notes, etc.

For example, here's the guide for 3D Tiles: https://github.com/AnalyticalGraphicsInc/3d-tiles/blob/master/CONTRIBUTING.md

CC #1002

Start by reviewing the linked 3D Tiles CONTRIBUTING.md guide and the glTF repository's current contributor guidance. Done means a short guide explains spec editing and defines consistent terminology for the runtime, converters, and implementation notes.

Written by the indexing model from the issue text.