add fog documentation

lib/paperclip/storage/fog.rb

 module Paperclip
   module Storage
+    # fog is a modern and versatile cloud computing library for Ruby.
+    # Among others, it supports Amazon S3 to store your files. In
+    # contrast to the outdated AWS-S3 gem it is actively maintained and
+    # supports multiple locations.
+    # 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 There are a few fog-specific options for
+    # has_attached_file, which will be explained using S3 as an example:
+    # * +fog_credentials+: Takes a Hash with your credentials. For S3,
+    #   you can use the following format:
+    #     aws_access_key_id: '<your aws_access_key_id>'
+    #     aws_secret_access_key: '<your aws_secret_access_key>'
+    #     provider: 'AWS'
+    #     region: 'eu-west-1'
+    # * +fog_directory+: This is the name of the S3 bucket that will
+    #   store your files.  Remember that the bucket must be unique across
+    #   all of Amazon S3. If the bucket does not exist, Paperclip will
+    #   attempt to create it.
+    # * +path+: This is the key under the bucket in which the file will
+    #   be stored. The URL will be constructed from the bucket and the
+    #   path. This is what you will want to interpolate. Keys should be
+    #   unique, like filenames, and despite the fact that S3 (strictly
+    #   speaking) does not support directories, you can still use a / to
+    #   separate parts of your file name.
+    # * +fog_public+: (optional, defaults to true) Should the uploaded
+    #   files be public or not? (true/false)
+    # * +fog_host+: (optional) The fully-qualified domain name (FQDN)
+    #   that is the alias to the S3 domain of your bucket, e.g.
+    #   'http://images.example.com'. This can also be used in
+    #   conjunction with Cloudfront (http://aws.amazon.com/cloudfront)
 
     module Fog
       def self.extended base