doc: Document cross compilation setup

Justfile

@@ -47,8 +47,6 @@ start-wineserver:
     fi
 
 setup-msvc:
-    rustup target add x86_64-pc-windows-msvc
-    rustup component add rust-src
     mkdir -p ./tools
     curl --fail -L https://github.com/Jake-Shadle/xwin/releases/download/{{ xwin_version }}/{{ xwin_prefix }}.tar.gz | tar -xzv -C ./tools --strip-components=1 {{ xwin_prefix }}/xwin
     ./tools/xwin --accept-license splat --include-debug-libs --output ./msvc

README.adoc

@@ -11,3 +11,29 @@
 :warning-caption: :warning:
 
 An engine plugin for Warhammer 40,000 Darktide that implements peer-to-peer networking.
+
+== Building
+
+> This assumes cross-compiling on a Linux build machine.
+> If you're on Windows, you're on your own. Shouldn't be too difficult, you're on native build anyways.
+
+=== Prerequisites
+
+* A reasonably recent version of LLVM, lld & Clang
+** A symlink `clang-cl` -> `clang` in `PATH`
+* cURL
+* Wine
+* Rust nightly toolchain, MSVC target and `rust-src`:
+** `rustup toolchain install nightly`
+** `rustup target add x86_64-pc-windows-msvc`
+** `rustup component add rust-src`
+
+=== Build
+
+You'll need to download the MSVC libraries once (their license forbids redistribution, even in something like a Docker image).
+
+With https://github.com/casey/just[just], simply run `just setup-msvc`, otherwise run the corresponding commands from `Justfile` manually. This will download the current Visual Studio Build Tools and corresponding libraries and headers into `./msvc`.
+
+Then run `just build`, which runs `cargo build` with the necessary toolchain variables set (see top of `Justfile`).
+
+The `.dll` in `target/x86_64-pc-windows-msvc/debug` can then be copied to the game's `binaries/plugins` folder. Remember to rename it so it has the required `_pluginw64_release.dll` suffix.