Update README to demonstrate passing an interpolation string to processing_image_url
Update README to demonstrate passing an interpolation string to processing_image_url

It wasn't immediately obvious to me (and maybe some others, see #118) that the string given to `processing_image_url` would be passed through the interpolated based on the previous example. This should make it clearer that you can include interpolation placeholders, making it easy to have per-style processing images.
matt0903/delayed_paperclip,jrgifford/delayed_paperclip