added documentation for s3 headers as proc

lib/paperclip/storage/s3.rb

@@ -41,7 +41,9 @@ module Storage
     # * +s3_protocol+: The protocol for the URLs generated to your S3 assets. Can be either
     #   'http' or 'https'. Defaults to 'http' when your :s3_permissions are :public_read (the
     #   default), and 'https' when your :s3_permissions are anything else.
-    # * +s3_headers+: A hash of headers such as {'Expires' => 1.year.from_now.httpdate}
+    # * +s3_headers+: A hash of headers or a Proc. You may specify a hash such as
+    #   {'Expires' => 1.year.from_now.httpdate}. If you use a Proc, headers are determined at
+    #   runtime. Paperclip will call that Proc with attachment as the only argument.
     # * +bucket+: 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. The bucket name will not be interpolated.