Add sample models attribution

test/integration/models/README.md

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+ATTRIBUTION
+-----------
+
+Some models in this folder come from the [Khronos gltf sample models repository](https://github.com/KhronosGroup/glTF-Sample-Models). They are:
+
+* AlphaBlendModeTest : Copyright 2018 Analytical Graphics, Inc. CC-BY 4.0 https://creativecommons.org/licenses/by/4.0/ Model and textures by Ed Mackey.
+
+* Boombox: Public Domain
+
+* Box/BoxTextured/BoxCompressed : Donated by Cesium for glTF testing. CC-BY 4.0 https://creativecommons.org/licenses/by/4.0/
+
+* BoxVertexColors: Public Domain
+
+* Duck: Copyright 2006 Sony Computer Entertainment Inc. Licensed under the SCEA Shared Source License, Version 1.0 (the "License"); you may not use this file except in compliance with [the License](https://web.archive.org/web/20160320123355/http://research.scea.com/scea_shared_source_license.html)
+
+* EnvironmentTest: https://stock.adobe.com/license-terms?prev_url=detail&comparison-full#enhanced-license-terms
+
+* MetalRoughSpheresNoTextures: Donated by Kirill Gavrilov for glTF testing. Public Domain.
+
+* MultiplePrimitives: Donated by Cesium for glTF testing. Please follow the Cesium Trademark Terms and Conditions. CC-BY 4.0 https://creativecommons.org/licenses/by/4.0/
+
+
+The rest of the models in this folder are property of Mapbox or they are under the public domain license.