Document cross-compiling for Windows using MXE.

This commits create a directory named 'win32' in the root directory with a README.md file and one script inside it. Right now the README.md contains full cross-compilation instructions using MXE. More can be added both in the directory and in the README.md file. The script collects all generated .exe and .dll files, plus all needed .dll files, plus the data directory, and creates a bundle (in another directory) that can be used directly in a Windows machine. Definitely people need to play around. No sound is tested yet and, in case of a crash, I also found that using the pre-built version of libsndfile from the website helped me. All is documented for now.