Windows installation instructions

Resolves #218

**Note:** I don't have the build environment installed locally (too painful to set up), so this PR was made completely _"blindly"_ and hasn't been tested at all yet — I did study the [DocBook reference/semantics](http://docbook.org/tdg/en/html/docbook.html) and XSL to hopefully create it correctly though.  The output is supposed to look almost identical to #218 (minor wording differences + additional paragraph for Cygwin/MingW32).