README.mingw improvement

[Xiaofan Chen]

I think some of the words in OpenOCD's "README.Windows" file
can be incorporated into libftdi's README.mingw file.

Ref: http://repo.or.cz/openocd.git/blob/HEAD:/README.Windows
Ref: http://developer.intra2net.com/git/?p=libftdi;a=blob;f=README.mingw

Idea 1: recommend MSYS2 since it has the infrastructure to build
libftdi, it also has the ready-made libftdi binary

Idea 2: provide the IgnoreHWSerNum registry setting hack idea

With newer version of libusb, another possibility is to use the
usbdk backend so as to use libusb/libftdi along with FTDI driver.