List proper storage option values for various storage backends (#112)

The "file" storage backend is actually called "filesystem" in
the code. Indicate the correct value to use in the readme to
save users frustration of resolving errors if they try "file"
as might be implied by the existing text.

Author: p

README.md

@@ -592,9 +592,9 @@ Storage
 
 Paperclip ships with 3 storage adapters:
 
-* File Storage
-* S3 Storage (via `aws-sdk-s3`)
-* Fog Storage
+* File Storage (`storage: :filesystem`)
+* S3 Storage (via `aws-sdk-s3`) (`storage: :s3`)
+* Fog Storage (`storage: :fog`)
 
 If you would like to use Paperclip with another storage, you can install these
 gems along side with Paperclip: