Add require: false to example code in README

Follow up of rubocop/rubocop-rspec#859.

This PR adds `require: false` to example code in README.

Author: koic

README.md

@@ -18,7 +18,7 @@ gem install rubocop-rails
 or if you use bundler put this in your `Gemfile`
 
 ```ruby
-gem 'rubocop-rails'
+gem 'rubocop-rails', require: false
 ```
 
 ## Usage