Document setup on Windows

[jscissr]

As noted by others (#70, #138), and I also experienced myself, there is currently not enough documentation on using gtk-rs on Windows.

After multiple failed attempts, wondering what I was supposed to do and googling error messages, I found this thread, which links to this file. I adapted and simplified these steps a bit, and can confirm that they actually work.

Let's avoid every newcomer wasting a lot of time by adding this to the documentation.

Fixes #70

[categulario]

I just recently had this same problem and your addition would have helped me but it also assumes the windows-gnu toolchain which is not always necessary what you want. For instance I solved my problems using the msvc toolchain.

Perhaps this topic requires its own page explaining the possibilities. (And I would happily contribute to it).