Add build instructions for Windows

[DiscoStarslayer]

Currently we have build instructions for macOS / Linux users but point Windows users to the AppVeyor builds.

This is great for the more common user, but if one is interested in developing/improving extract-iso it can be helpful to have build instructions.

[StrikerX3]

On Windows you'll need CMake and either Visual Studio or MSYS2 MinGW.

cmake -G "Visual Studio 15 2017" will generate a Project.sln in the folder. You can use cmake -G "Visual Studio 15 2017 Win64" to make project that builds a 64-bit executable.
If you're using MSYS2 MinGW, simply run cmake ., which will generate Makefiles for the project. Use make to build it.

[JayFoxRox]

To my knowledge, MinGW / MSYS makefiles need another makefile generator? Also has some extra steps to install these tools (not as easy as on Linux or macOS).

[StrikerX3]

My MSYS2 MinGW environment is already set up to work with XQEMU and other projects, so I likely have installed any necessary prerequisites. As for the extra steps, I assume that users will know how to set their environment up if they don't already have one. In any case, we may refer them to the respective documentation.