Revise some renamings of NOTES and README files

Revise some renamings of NOTES and README files

Some of the notes and readme files have been converted to markdown
format recently and renamed during this process. While adding the
.md extension was a natural step, switching to mixed cases was not
a change to the better, it gives them a ragged appearance:

     NOTES.ANDROID  => NOTES-Android.md
     NOTES.DJGPP    => NOTES-DJGPP.md
     NOTES.PERL     => NOTES-Perl.md
     NOTES.UNIX     => NOTES-Unix.md
     NOTES.VMS      => NOTES-VMS.md
     NOTES.VALGRIND => NOTES-Valgrind.md
     NOTES.WIN      => NOTES-Windows.txt
     README.ENGINE  => README-Engine.md
     README.FIPS    => README-FIPS.md

Moreover, the NOTES-Windows.txt file is the only file which has been
converted to markdown but has received a .txt file extension.
This doesn't make sense, because the OpenSSL users on Windows will
need to read the other markdown documents as well. Since they are
developers, we can trust them to be able to associate their favorite
editor with the .md extension.

In fact, having a comment at the beginning of the file saying that it
is in markdown format but we didn't dare to add the correct extension
in order not to overwhelm our Windows users can be interpreted either
as unintentionally funny or disrespectful ;-)

This commit suggests the following more consistent renaming:

     NOTES.ANDROID  => NOTES-ANDROID.md
     NOTES.DJGPP    => NOTES-DJGPP.md
     NOTES.PERL     => NOTES-PERL.md
     NOTES.UNIX     => NOTES-UNIX.md
     NOTES.VMS      => NOTES-VMS.md
     NOTES.VALGRIND => NOTES-VALGRIND.md
     NOTES.WIN      => NOTES-WINDOWS.md
     README.ENGINE  => README-ENGINES.md
     README.FIPS    => README-FIPS.md

(note the plural in README-ENGINES, anticipating a README-PROVIDERS)

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14042)