[docs] Adding a section about 'url_with_processing.'

On a CMS that I host on Heroku, my users were uploading animated gifs, which could not be resized within the 25s Heroku timeout.  delayed_paperclip made it easy to move the  resizing into a worker thread.  

However, then I had problems with giving effective feedback to the users while the image was being resized.  So, I added the "x_processing" booleans to the models, which worked well in the admin area, but the "missing.png" would get stuck in the public facing caching layer.

So I dug into the code and discovered the undocumented 'url_with_processing' configuration -- which, enables you to use the method called ".processing?", but turn off the "missing.png" piece.