Provide instructions for Windows systems

This doesn't mean that there is an official Windows support; prebuilt packages (with e.g. Cygwin, GS and Java) will not be provided (as I don't have a chance to test them, so it's pointless so ship something untested).

However, being GS and Java cross-platform, there should be in principle no obstacle in running pdf2archive in Windows (once one has a Bash shell). Additionally, given that I've put up pdf2archive right because Linux is discriminated in the availability of free PDF/A-1B converting solutions, it would not be nice to "discriminate back" Windows.

Possible solutions

• For Windows 10 on
  An option would be to use the WSL. Checks are needed.
• For older versions
  I think we need something like Cygwin. Users can be instructed to open a Cygwin terminal and run pdf2archive as you would do on Linux, or we can provide them with a .bat wrapper. Checks are needed.

TODO

• Find someone willing to test on Windows, as I don't have a Windows box available (and I don't plan to have one).
• Test the solution with WSL on Windows 10.
• Test the solution with Cygwin, both with and without a .bat.
• Provide instructions for installing Ghostscript, Java and a WSL Linux distro (or Cygwin).

Start by testing the pdf2archive entry point on Windows 10 through WSL and on older Windows through Cygwin, using the listed Ghostscript and Java dependencies. Check both direct shell usage and the proposed .bat wrapper, then document installation and running steps for each supported setup.