Update cross compilation section of README

[elieux]

I noticed the cross compilation section of README is a bit out of date in few regards. I would prepare a pull request, but I'm currently having some problems with GitHub's UI. If desired, I can make the changes myself and send a patch, or deal with the UI and send a PR.

In addition to having the development tool chain installed (GCC), you also need to install your platform's mingw32 cross compilation package.

Since mingw-w64 is later mentioned as an alternative, it'd be more correct to just say mingw here (but see point below).

On Arch, mingw32 is installed by running pacman -S mingw32-gcc

Arch Linux has deprecated mingw32 and now distributes mingw-w64. See list of mingw packages in their default repos. I suggest making mingw-w64 the default choice for all platforms.

You can add this to your .profile to avoid the repitition.

Typo.