Docs: building on Windows with MSVC, GNU, or GNU with bundled LLD (refs #1886)

CONTRIBUTING.md

@@ -13,6 +13,37 @@ cargo test --workspace
 cargo xtask ci                           # what every commit must pass
 ```
 
+### Windows
+
+The MSVC toolchain, rustup's default on Windows, is what the releases use: with the Visual Studio
+C++ build tools installed, the commands above work as they are.
+
+The GNU toolchain (`x86_64-pc-windows-gnu`) needs a current mingw-w64 GCC and binutils first on
+`PATH`. They provide the linker and the Windows import libraries, such as `libshlwapi.a` for
+opening links from the app. With [MSYS2](https://www.msys2.org/):
+
+```powershell
+C:\msys64\usr\bin\pacman.exe -S --needed mingw-w64-x86_64-gcc
+$env:PATH = "C:\msys64\mingw64\bin;$env:PATH"
+cargo +stable-x86_64-pc-windows-gnu run -p effectcraft -- --demo
+```
+
+An older MinGW earlier on `PATH` can break the build: CodeBlocks' GCC 8.1 fails to link
+`boa_engine` ([#1882](https://github.com/storytold/effectcraft/issues/1882)).
+
+Rust's bundled LLD and self-contained libraries don't include every import library EffectCraft
+needs, so on their own they stop with `unable to find library -lshlwapi`
+([#1886](https://github.com/storytold/effectcraft/issues/1886)). To link that way, add mingw-w64's
+libraries to the search path in your own Cargo config (`%USERPROFILE%\.cargo\config.toml`):
+
+```toml
+[target.x86_64-pc-windows-gnu]
+linker = "rust-lld"
+rustflags = ["-C", "link-self-contained=yes", "-L", "native=C:/msys64/mingw64/lib"]
+```
+
+The `Windows toolchains` workflow builds the app and the CLI each of these three ways every night.
+
 ## Ground rules
 
 - **Clean room.** Work from After Effects' public documentation and its observable behaviour only.