Spec contributor's guide

[pjcozzi]

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

[pjcozzi]

Example: avoid multiple inheritance in JSON schema

CC @javagl
CC #1144

[pjcozzi]

Well said by @lexaknyazev:

We usually regard schemas' descriptions as more formal and README.md as more approachable.

CC #1229 (comment)

[donmccurdy]

It seems like various authors have submitted extension specs and schema more or less successfully - is this guide still needed, or should we close this?

[pjcozzi]

Still seems valuable given extension inconsistencies in #1491, right?