Add ktx

ktx/external/basisu/webgl/gltf/README.md

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+## Credits
+
+* Contributors:
+  * [Don McCurdy](https://www.donmccurdy.com)
+  * [Austin Eng](https://github.com/austinEng)
+  * [Shrek Shao](https://github.com/shrekshao)
+* Made with [three.js](https://threejs.org/).
+* Thanks to [AGI](http://agi.com/) for providing the glTF model.