format of win/README-Windows-jp.txt

[Tatsuro M. <tma...@ya...>]

When upgrading from gnuplot-6.0.0 to gnuplot-6.0.1, win/README-Windows-ja.txt changed from Shift_JIS to UTF-8.
The win/README-Windows.txt file that appears at the beginning of the installer was garbled in Japanese environment.
After converting the file to Shift_JIS, the characters are no longer garbled.
If win/README-Windows-ja.txt is to be maintained in UTF-8 in the gnuplot repository, 
it may be necessary to include a process in the Makefile (config/mingw/Makefile) to convert the code to Shift_JIS when making the file.

Tatsuro