Add to README how to setup or disable Amazon S3

What happened just before the problem occurred

Trying to get staging instance running, found hidden exception:

Errno::ENOENT (No such file or directory @ rb_sysopen - config/amazon_s3.yml):                                                                                                   
  app/controllers/images_controller.rb:51:in `block in create'                                                                                                                   
  app/controllers/images_controller.rb:50:in `create'