add cesium_primitive_outline to extensions doc

src/SharpGLTF.Core/README.md

@@ -66,6 +66,8 @@ It also handles the way a mesh is brought from its local space to world space, i
 - [ ] [EXT_lights_image_based](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/EXT_lights_image_based)
 - [x] [EXT_texture_webp](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/EXT_texture_webp)
 - [ ] [ADOBE_materials_thin_transparency](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/ADOBE_materials_thin_transparency)
+- [x] [CESIUM_primitive_outline](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/CESIUM_primitive_outline)
+