Update rubocop command option in README.md

Follow up #55 (comment).

This PR updates `rubocop` command option in README.md.

`rubocop --rails` command option is required while `rubocop` command supports `--rails` option.

Author: koic

README.md

@@ -38,9 +38,11 @@ cops together with the standard cops.
 ### Command line
 
 ```sh
-rubocop --require rubocop-rails
+rubocop --require rubocop-rails --rails
 ```
 
+Note: `--rails` option is required while `rubocop` command supports `--rails` option.
+
 ### Rake task
 
 ```ruby