More detailed instructions for Windows

These instructions are not definitive, I'm still experiencing an error when "mingw32-make install":
mingw32-make: *** No rule to make target '_obj/wx-wx.dll', needed by 'C:\Go\/pkg/windows_386/swig/wx-wx.dll'.  Stop.

Probably this way to generate wxWidgets binding for Go is obsolete, I think I will perhaps try later another way.