README.Cygwin: Rename -> README.Cygwin.md. Replace the contents of this file; its existing contents were horribly out-of-date. README.MS-VisualC.md: README.MS-Windows.md: Update links. NEWS.md: Announce that 32-bit Cygwin is no longer supported.
README.MS-VisualC.md: Document how to use the 64-bit version of MSVC. Delete references to the 64-bit version not being supported.
README.MS-VisualC: As above. README.md: Conform to the above change.