Add README

Author: thatcosmonaut

README.txt

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+SDL_gpu_shadercross
+
+This is a library for translating shaders to different formats, intended for use with SDL's GPU API.
+It takes SPIRV or HLSL as the source and outputs DXBC, DXIL, SPIRV, MSL, or HLSL.
+
+This library can perform runtime translation and conveniently returns compiled SDL GPU shader objects from HLSL or SPIRV source.
+This library also provides a command line interface for offline translation of shaders.
+
+For SPIRV translation, this library depends on SPIRV-Cross: https://github.com/KhronosGroup/SPIRV-Cross
+For compiling to DXIL, dxcompiler.dll and dxil.dll are required (or your platform's equivalent).
+For compiling to DXBC, d3dcompiler_47 is shipped with Windows. Other platforms require vkd3d-utils.
+
+This library is under the zlib license, see LICENSE.txt for details.