[m-rev.] for review: update documentation for Cygwin

[Peter Wang]

On Sun, 13 Aug 2023 14:34:12 +1000 Julien Fischer <jfischer at opturion.com> wrote:
> 
> For review by anyone.
> 
> ---------------------
> 
> Update documentation for Cygwin.
> 
> README.Cygwin:
>      Rename -> README.Cygwin.md.
> 
>      Replace the contents of this file; its existing contents were
>      horribly out-of-date.
> 

That's fine.

Peter