Added Windows binaries link to README

I've started playing around with rasterio on my mac, and on my Linux server, but assumed from the README that I couldn't use it on my Windows PC at work - which would be a showstopper for me.

I then found that Christoph Gohlke's wonderful page of binary Python packages for Windows has rasterio downloads, and I downloaded it and it worked perfectly. I thought that should be in the README, so I've added a link to it.