Update Windows build guidance

The intent of this PR is update README.windows.md with known-good-as-of-date instructions. Key changes include:

1. promoting MSYS2 from experimental to preferred status
2. placing MSYS in a fallback position for users who cannot or prefer not to use MSYS2.
3. simplifying the shell environment setup (no changes to fstab; minimal path to avoid shadowing)
4. relying on official Python 2.x build for Windows makes MSYS[2] python redundant
5. note an idiosyncracy of the currently used version of OpenBLAS which requires a small detour

Feedback on content and style are all welcome. Most of all, thanks to everyone who helped me get comfortable with building Julia on Windows.