Make Rails version compatibility clearer

Fixes #1

Author: lsegal

README.md

@@ -1,8 +1,8 @@
 # Amazon DynamoDB Session Store
 
 The **Amazon DynamoDB Session Store** handles sessions for Ruby web applications
-using a DynamoDB backend. The session store is compatible with Rails and other
-Rack based frameworks.
+using a DynamoDB backend. The session store is compatible with Rails (3.x or 4.x)
+and other Rack based frameworks.
 
 ## Installation
 
@@ -168,4 +168,4 @@ on the {AWS::SessionStore::DynamoDB::Errors::BaseHandler} class for more
 details.
 
 [1]: http://rubydoc.org/gems/aws-sessionstore-dynamodb/frames
-[2]: http://rubydoc.org/gems/aws-sessionstore-dynamodb/AWS/SessionStore/DynamoDB/Configuration#initialize-instance_method
+[2]: http://rubydoc.org/gems/aws-sessionstore-dynamodb/AWS/SessionStore/DynamoDB/Configuration#initialize-instance_method