Clarify AWS-SDK versions in README

Add context from:
https://github.com/thoughtbot/paperclip/issues/2021

[ci skip]

README.md

@@ -554,7 +554,7 @@ Storage
 Paperclip ships with 3 storage adapters:
 
 * File Storage
-* S3 Storage (via `aws-sdk`)
+* S3 Storage (via `aws-sdk` or `aws-sdk-v1`)
 * Fog Storage
 
 If you would like to use Paperclip with another storage, you can install these
@@ -582,7 +582,11 @@ You may also choose to store your files using Amazon's S3 service. To do so, inc
 the `aws-sdk` gem in your Gemfile:
 
 ```ruby
-gem 'aws-sdk'
+gem 'aws-sdk', '>= 2.0.0' # If using paperclip `master`
+```
+or
+```ruby
+gem 'aws-sdk-v1' # If using paperclip <= v4.3.1
 ```
 
 And then you can specify using S3 from `has_attached_file`.