SheenTestGrid added

Grid of spheres over a checkered backdrop to test Sheen rendering.

Requested in issue #30

Models/SheenTestGrid/README.body.md

@@ -0,0 +1,18 @@
+## Screenshot
+
+![Screenshot from glTF Sample Viewer](screenshot/screenshot_Large.jpg)
+<br/>_Screenshot from [glTF Sample Viewer](https://github.khronos.org/glTF-Sample-Viewer-Release/) using the Environment "Footprint Court"._
+
+## Description
+
+This model tests sheenColorFactor versus sheenRoughnessFactor using the extension [KHR_materials_sheen](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_sheen). 
+
+The baseColorFactor for all spheres is 0.5 blue, and the sheenColorFactor varies from 0 black to 0,1,1 cyan. 
+
+The sheen extension has been explicitly added to each of the sixteen materials, even when the sheenColorFactor is set to black and therefore the extension would usually be omitted.
+
+![Screenshot from glTF Sample Viewer](screenshot/sheen-sheenColor-sheenRough.jpg)
+<br/>_Screenshots from [glTF Sample Viewer](https://github.khronos.org/glTF-Sample-Viewer-Release/) using the Environment "Studio Neutral" and showing debug views Sheen, Sheen Color, and Sheen Roughness._
+
+![Screenshot from glTF Sample Viewer](screenshot/screenshot_Punctual.jpg)
+<br/>_Screenshot from [glTF Sample Viewer](https://github.khronos.org/glTF-Sample-Viewer-Release/) with the Punctual Lighting option, which uses two directional lights from opposing angles._