README.md: Add cross-compiling guide for macOS to Windows

Signed-off-by: Lilis Iskandar <[email protected]>

Author: veeableful

README.md

@@ -117,6 +117,16 @@ Cross-compiling
 5. Before running the program, you need to put `SDL2.dll` from the [SDL2 runtime package](http://libsdl.org/download-2.0.php) (For others like *SDL_image*, *SDL_mixer*, etc.., look for them [here](https://www.libsdl.org/projects/)) for Windows in the same folder as your executable.
 6. Now you should be able to run the program using Wine or Windows!
 
+### macOS to Windows
+
+1. Install [Homebrew](https://brew.sh)
+2. Install MinGW through Homebrew via `brew install mingw-w64`
+3. Download the SDL2 development package for MinGW [here](http://libsdl.org/download-2.0.php) (and the others like *SDL_image*, *SDL_mixer*, etc.. [here](https://www.libsdl.org/projects/) if you use them).
+4. Extract the SDL2 development package and copy the `x86_64-w64-mingw` folder inside recursively to the system's MinGW `x86_64-w64-mingw32 folder. You may also do the same for the `i686-w64-mingw32` folder. The path to MinGW may be slightly different but the command should look something like `cp -r x86_64-w64-mingw32 /usr/local/Cellar/mingw-w64/5.0.3/toolchain-x86_64`.
+5. Now you can start cross-compiling your Go program by running `env CGO_ENABLED=1 CC=x86_64-w64-mingw32-gcc GOOS=windows CGO_LDFLAGS="-L/usr/local/Cellar/mingw-w64/5.0.3/toolchain-x86_64/x86_64-w64-mingw32/lib -lSDL2" CGO_CFLAGS="-I/usr/local/Cellar/mingw-w64/5.0.3/toolchain-x86_64/x86_64-w64-mingw32/include -D_REENTRANT" go build -x main.go`. You can change some of the parameters if you'd like to. In this example, it should produce a `main.exe` executable file.
+6. Before running the program, you need to put `SDL2.dll` from the [SDL2 runtime package](http://libsdl.org/download-2.0.php) (For others like *SDL_image*, *SDL_mixer*, etc.., look for them [here](https://www.libsdl.org/projects/)) for Windows in the same folder as your executable.
+7. Now you should be able to run the program using Wine or Windows!
+
 FAQ
 ===
 __Why does the program not run on Windows?__