doc: add BUILDING.md with Windows instructions

thermion_dart/BUILDING.md

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+# Instructions for building Filament
+
+Below are instructions for building the Filament (currently pinned to v1.51.2) for each of the target platforms.
+
+This is only for developers extending the Thermion package itself; if you are simply using Thermion as a dependency in your `pubspec.yaml`, you can ignore this.
+
+## Windows
+
+- Install Visual Studio 2022
+- Open Developer Command Prompt 
+
+> where cmake
+
+(If multiple entries appear, you'll need to refer to the VS2022 version explicitly)
+
+> mkdir out; 
+> cd out
+> cmake -DUSE_STATIC_CRT=OFF ..
+> cmake --build . --config Release
+> cmake --build . --target tinyexr --config Release
+> cmake --build . --target imageio --config Release