Raw documentation for the bdist_wininst postinstallation script.

Raw documentation for the bdist_wininst postinstallation script.
People are already using it, so these docs are certainly better than
no docs at all. Markup is mostly missing and the layout is probably
ugly, but this can be fixed later.

Question: there are references to MS docs for the CSIDL_... constants
and the IShellLink interface. Are these pointers sufficient, or should
the MS docs reworded and repeated here?