Add CONTRIBUTING.md with CLA and AI text

[KhronosWebservices]

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

[gmlueck]

When I follow the link at (https://www.khronos.org/cla/KhronosGroup/SYCL-Docs), it says this under Outbound License Terms:

Specifications. Khronos licenses its entire specifications under the terms of the Khronos Specification Copyright License at: www.khronos.org/legal/Khronos_Specification_Copyright_License_Header.

However, the published SYCL specification also lists its license, and that is not exactly the same as the content of "Khronos_Specification_Copyright_License_Header". You can see the published license at the very beginning of the spec:

https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html

Should we do something to resolve this difference?

[KhronosWebservices]

@oddhack can you take a look?

@gmlueck Might be good to open this as a separate issue, as it doesn't have any bearing on this PR specifically. The SYCL Docs repo is under the Mix Repo CLA and this PR only clarifies that fact, and the AI contributor paragraph.

[KhronosWebservices]

@gmlueck Okay to merge this PR now?

[gmlueck]

@TApplencourt this is up to you as chair. I have no concerns.

@KhronosWebservices: We normally approve PRs to merge in the weekly WG call, but the agenda for our last call did not have time for this. Is this something that is urgent, or can it wait for next week's call?

[KhronosWebservices]

Thanks @gmlueck Can wait til next week.

[TApplencourt]

I was waiting for @oddhack feedback too. Don't want to merge conflicting-lawyers-thingy : )
But because it's a Khronos PR, I have no objection neither

[KhronosWebservices]

This PR doesn't touch licensing, just adding pre-disclosures for the existing license and adds the new required AI text. The CLA is already in place and being used.

[TApplencourt]

👍🏽 we will follow up on another issue then!
Thanks

[KhronosWebservices]

Thank you everyone!