Add windows/windows64 build support guide.

[luckcolors]

How can i build on windows without having problem on cygwin? Here's my error log(translated from italian):
Luckcolors@Luckcolors-PC ~
$ go get github.com/abh/geodns
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/../../../../x86_64-pc-cygwin/bin/ld: cannot find -lmingwex
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/../../../../x86_64-pc-cygwin/bin/ld: cannot find -lmingw32
collect2: error: ld returned 1 exit status

[arulrajalivi]

I am seeing the same issue
any help?

[abh]

I've no idea, sorry. I don't know anything about windows. :-/ Patches to abh/geoip that fixes this would be very welcome.