Ship with native build instructions for windows
In #4520 (moved), we removed our mingw native compilation instructions, since they were out-of-date, and out-of-sync with our actual process. We should replace them with something better.
It's not likely to be our exact build process, though: our own binaries are cross-compiled using gitian in order to achieve reproducible builds. We should link to that, but also include a cleaned-up version for people who want to work natively on Windows.