Documentation: add simple instructions on how to cross-compile for Windows from normal operating systems

Documentation/cross-compiling-for-windows-with-wine.md

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+# Cross-compiling for Windows with Wine
+
+This can be useful to test engine in Wine without using virtual machines or dual-booting to Windows.
+
+0. Clone and install https://github.com/mstorsjo/msvc-wine (you can skip CMake part)
+1. Set environment variable WINE_MSVC_PATH to the path to installed MSVC toolchain
+2. Pre-load wine: `wineserver -k; wineserver -p; wine64 wineboot`
+3. Run `./waf configure -T <build-type> --enable-wine-msvc --sdl2=../SDL2_VC`. Configuration step will take more time than usual.
+4. Follow other typical steps to build from console, except keep in mind you're in decent Operating System now.