Update README.md with info on using robot.rb

Added instructions to push an image to a badge using robot.rb. robot.rb requires that the ImageMagick libraries and gems are installed.
One idea, which is more of a code change than documentation change, would be to add `resize_to_fit` and `extent` so that the image sent to the badge is always the correct size.  I do something like that for my own hacking: https://github.com/bettse/fs_badge/blob/master/webservice/app.rb#L186