[Fix #56] Add a reference to rubocop-rails_config gem in README.md

This PR adds a reference to `rubocop-rails_config` gem in README.md

Fixes #56.
Follow up #57 (comment).
Follow up rubocop/rubocop#5976 (comment).

Author: koic

README.md

@@ -5,6 +5,8 @@
 
 A [RuboCop](https://github.com/rubocop-hq/rubocop) extension focused on enforcing Rails best practices and coding conventions.
 
+Note: This repository manages rubocop-rails gem (>= 2.0.0). rubocop-rails gem (<= 1.5.0) has been renamed to [rubocop-rails_config](https://rubygems.org/gems/rubocop-rails_config) gem.
+
 ## Installation
 
 Just install the `rubocop-rails` gem