Readme For Windows Users

Windows users don't have any real way to see a solid fix for the infamous "spoofing" error - `Validation failed: Upload file has an extension that does not match its contents.`

Having fixed this ourselves, I provided more in-depth information for your users. This *should* help anyone developing on Windows. I also tidied up the ReadMe in general (adding breaker lines to denote sections etc)

BTW here is a pic of me & friend your Stockholm office:
![thoughtbot](https://cloud.githubusercontent.com/assets/1104431/4530754/dc9b8f1e-4d83-11e4-8d84-50857eebfddd.JPG)