chore: Add Legal mandated AI block to CONTRIBUTING (#42)

Author: KhronosWebservices

CONTRIBUTING.md

@@ -1,3 +1,8 @@
+<!--
+SPDX-FileCopyrightText: 2026 The Khronos Group Inc.
+SPDX-License-Identifier: CC-BY-4.0
+-->
+
 # SPIR-V Guide Contributing
 
 While open for contributions from all, there are a few contribution rules in place.
@@ -15,4 +20,15 @@ The main design goal for the SPIR-V Guide is to be "lean" and prevent any duplic
 
 ## Images
 
-All images must be no wider than 1080px. This is to force any large images to be resized to a more reasonable size.
+All images must be no wider than 1080px. This is to force any large images to be resized to a more reasonable size.
+
+## 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.