Add ktx

ktx/external/dfdutils/vulkan/README.md

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+<!-- Copyright 2023 The Khronos Group Inc. -->
+<!-- SPDX-License-Identifier: Apache-2.0 -->
+
+# Why Vulkan Header Files Here?
+
+The `vulkan_core.h` included here is custom generated from the Vulkan
+Registry's `vk.xml` in order to include the format enumerators for the
+ASTC 3d formats. We need to support these formats because there are OpenGL implementations even though, as yet, there are no Vulkan implementations.
+
+See `./build_custom_vulkan_core` for instructions on generating the file.
+
+`vulkan_core.h` is used to generate the switch bodies `dfd2vk.inl` and `vk2dfd.inl`.
+
+## To Do
+
+Generate the switch bodies from `vk.xml` instead.