Added Eric's anisotropy disc test model.

Models/AnisotropyDiscTest/README.body.md

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+## Screenshot
+
+![screenshot](screenshot/screenshot-large.png)
+
+## Description
+
+This model tests the main texturemap for [`KHR_materials_anisotropy`](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_anisotropy).  It has the following features:
+
+- An array of base material roughness values are tested with the same texturemap each time.
+
+- The upper-left quadrant tests a disc-shaped anisotropic pattern at a strength of about 3%.
+
+- The upper-right quadrant tests a disc-shaped anisotropic pattern at a strength of about 10%.
+
+- The lower-right quadrant tests a disc-shaped anisotropic pattern at a full strength.
+
+- The lower-left quadrant tests a disc-shaped anisotropic pattern with a gradient strength, starting at zero around the edges and increasing to full strength in the center.
+
+Per the anisotropy specification, anisotropic strength increases the material's roughness along a particular direction.  Therefore, zero base roughness with full-strength anisotropy displays the most pronounced result.  Full 1.0 base roughness cannot increase more, and does not show any effects from anisotropy.