Add CONTRIBUTING.md with CLA and AI text

[KhronosWebservices]

Adds CONTRIBUTING.md with legal mandated AI-assisted-contributions disclosure
and the Khronos Open Source CLA pre-disclosure.

[CLAassistant]

All committers have signed the CLA.

[javagl]

*condescendingly wiggles index finger* "Ah ah ah, you didn't say the magic word".

I assume that it should be merged, even though the CLA is not signed (formalities...), and will do that later, unless someone disagrees.

[KhronosWebservices]

CLA signed. Took awhile to get to them all. :)

[javagl]

The ""good"" thing here is: You can have a look at all your PRs in a week or so. And by identifying the ones that are not merged yet, you also identify the repos that are essentially dormant or abandoned (and my gut feeling is that there are many of them...)

[KhronosWebservices]

The ""good"" thing here is: You can have a look at all your PRs in a week or so. And by identifying the ones that are not merged yet, you also identify the repos that are essentially dormant or abandoned (and my gut feeling is that there are many of them...)

The script that creates these PRs (not AI) also produces a nicely formatted checklist in Markdown, making it easy to review the PRs. Thank you for your support on this.