This PR adds a CONTRIBUTING file which includes a legally mandated AI Contributions block of text (#187)

CONTRIBUTING.adoc

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+////
+SPDX-FileCopyrightText: 2026 The Khronos Group, Inc.
+SPDX-License-Identifier: CC-BY-4.0
+////
+
+= Contributing
+
+
+== AI-Assisted Contributions
+
+By submitting a Contribution to this repository, you additionally represent
+that, to the extent any of Your Contributions were developed with the assistance
+of artificial intelligence tools or AI-generated code, You have exercised
+sufficient review, judgment, and creative direction over such tools and
+resulting material to reasonably consider it Your original creation, and You are
+not aware of any third-party license, intellectual property claim, or other
+restriction arising from such use that is associated with any part of Your
+Contribution or use thereof.