windows build comments with winbuild repository link

This PR adds a pointer to the windows build script aimed at addressing the needs of https://github.com/matplotlib/matplotlib/issues/1717

I removed the "If you are a Windows user, you can ignore this because we ..." sentences because if you are reading the section titled "Build..." you can't ignore them.  The MPL recommended approach is (roughly speaking) to read and follow the install recommendation on Windows and don't read the build details.