add some documentation about the 3D Tiles glTF test fixtures, and how to validate them

tests/SharpGLTF.Cesium.Tests/testfixtures/README.md

@@ -1 +1,14 @@
-3D Tiles Test fixtures are obtained from https://github.com/CesiumGS/3d-tiles-validator/tree/main/specs/data/gltfExtensions
+# Test fixtures
+
+This directory contains glTF files used for testing 3D Tiles functionality.
+
+3D Tiles Test fixtures are obtained from https://github.com/CesiumGS/3d-tiles-validator/tree/main/specs/data/gltfExtensions
+
+## Validating
+
+The files can be validated using the 3D Tiles Validator:
+
+```
+$ git clone https://github.com/CesiumGS/3d-tiles-validator
+$ npx ts-node  ./3d-tiles-validator/src/main.ts --tileContentFile test.gltf
+```