documentation for including aws-sdk in Gemfile

README.md

@@ -164,9 +164,14 @@ file at
 _**NOTE**: This is a change from previous versions of Paperclip, but is overall a
 safer choice for the default file store._
 
-You may also choose to store your files using Amazon's S3 service. You can find
-more information about S3 storage at the description for
-[`Paperclip::Storage::S3`](http://rubydoc.info/gems/paperclip/Paperclip/Storage/S3).
+You may also choose to store your files using Amazon's S3 service. To do so, include
+the `aws-sdk` gem in your Gemfile:
+
+    gem 'aws-sdk'
+
+And then you can specify using S3 from `has_attached_file`. 
+You can find more information about configuring and using S3 storage in
+[the `Paperclip::Storage::S3` documentation](http://rubydoc.info/gems/paperclip/Paperclip/Storage/S3).
 
 Files on the local filesystem (and in the Rails app's public directory) will be
 available to the internet at large. If you require access control, it's

lib/paperclip/storage/s3.rb

@@ -2,6 +2,9 @@ module Paperclip
   module Storage
     # Amazon's S3 file hosting service is a scalable, easy place to store files for
     # distribution. You can find out more about it at http://aws.amazon.com/s3
+    #
+    # To use Paperclip with S3, include the +aws-sdk+ gem in your Gemfile:
+    #   gem 'aws-sdk'
     # There are a few S3-specific options for has_attached_file:
     # * +s3_credentials+: Takes a path, a File, or a Hash. The path (or File) must point
     #   to a YAML file containing the +access_key_id+ and +secret_access_key+ that Amazon