[Distutils] Including documentation into a Windows installer

[Martin v. Loewis]

> Maybe you need to write a special install_docs class to do this...

Indeed, that is the solution. Rene Liebscher uses an advanced
install_data command for PyOpenGL; I incorporated that into PyXML.

Still, I'd prefer if distutils had direct support for documentation
files, and would always find the right location for them on the
different systems.

Regards,
Martin