Documentation update about Cygwin and MinGW

[Mingye Wang]

While reading the readline doc[1] I noticed that the page currently
recommends going to MinGW for a build of readline. The issue is that
"MinGW" has been dead for a very long time, to the point that the
linked website no longer works.

  [1]: https://tiswww.case.edu/php/chet/readline/rltop.html

MinGW is currently succeeded by "Mingw-w64". It continues to act as a
lightweight environment for GNU toolchains to work on Windows, but the
project itself only manages the glue that makes them work, not an
entire distribution. The libraries to be run under MinGW64 are built
by "distributions" like Cygwin, MSYS2, and <insert Linux distribution
here> using these toolchains. On Cygwin, the packages are
mingw64-{i686,x86_64}-readline (both version 7.0). The related MSYS2
has mingw-w64-{i686,x86_64}-readline at 8.0.

In conclusion, I recommend replacing the "or MinGW, which currently
has packages for readline-5.2" part with something along the lines of
"for a version that runs independently of Cygwin, see the
mingw64-{i686,x86_64}-readline packages."

Thanks,
Mingye Wang (Artoria2e5)