doc: add instructions to build for Windows on macOS (#216)

x/examples/fyne-tools/README.md

@@ -6,6 +6,8 @@ To run:
 go run github.com/Jigsaw-Code/outline-sdk/x/examples/fyne-tools@latest
 ```
 
+## Android
+
 Package for Android and install on emulator or device:
 ```
 go run fyne.io/fyne/v2/cmd/fyne package -os android && adb install Net_Tools.apk
@@ -14,6 +16,35 @@ go run fyne.io/fyne/v2/cmd/fyne package -os android && adb install Net_Tools.apk
 Note: the generated APK is around 85MB.
 
 
+## Windows
+
+If you are on Windows, you can just use the regular `go build` or `go run` command.
+
+Because the app uses cgo, we need to cross-compilation tools to build from other platforms.
+
+If you are on macOS, you can build the Windows app with [MinGW-x64](https://www.mingw-w64.org/).
+
+First install MinGW-w64. MacPorts is the [official channel](https://www.mingw-w64.org/downloads/#macports):
+
+```
+sudo port install x86_64-w64-mingw32-gcc
+```
+
+With Homebrew (unofficial):
+
+```
+brew install mingw-w64
+```
+
+Build the app (64-bit):
+
+```
+GOOS=windows GOARCH=amd64 CGO_ENABLED=1 CC="x86_64-w64-mingw32-gcc" go build .
+```
+
+The first build will take minutes, since there's a lot of platform code to be built.
+Subsequent builds will be incremental and take a few seconds.
+
 ## Screenshots
 
 <img width="400" alt="image" src="https://github.com/Jigsaw-Code/outline-sdk/assets/113565/8cead9da-461e-41c8-8ce3-f263d77c6ee8">