The intent of this PR is update README.windows.md with known-good-as-of-date instructions.

Key changes include:

1. stating which versions of Windows are supported as build platforms
2. promoting MSYS2 from experimental to preferred status
3. placing MSYS in a fallback position for users who cannot or prefer not to use MSYS2.
4. simplifying the shell environment setup (no changes to fstab; minimal path to avoid shadowing)
5. relying on official Python 2.x build for Windows makes MSYS[2] python redundant
6. note an idiosyncracy of the currently used version of OpenBLAS which requires an extra `make`
7. updating the paths for dependnecies, where these have changed on SourceForge

Feedback on content and style are all welcome.
Most of all, thanks to everyone who helped me get comfortable with building Julia on Windows.
Merge pull request #6019 from cmundi/master

Update Windows build guidance