README: Add instructions for building on windows.

Getting rust to build on windows can be a bit annoying in setting up all the toolchains and whatnot. The whole process is made much easier by using msys2 so let's document that prominently right on the README.
Yep, it is currently required on 64-bit windows since `uname -a` always returns `x86_64`. Without the flag, `configure` recognizes `CFG_BUILD := x86_64-pc-mingw32` on `mingw32_shell.bat`.