Document .rubocop.yml in rails default file folder structure

review changes updated

guides/source/getting_started.md

@@ -191,6 +191,7 @@ of the files and folders that Rails creates by default:
 |.dockerignore|This file tells Docker which files it should not copy into the container.|
 |.gitattributes|This file defines metadata for specific paths in a git repository. This metadata can be used by git and other tools to enhance their behavior. See the [gitattributes documentation](https://git-scm.com/docs/gitattributes) for more information.|
 |.gitignore|This file tells git which files (or patterns) it should ignore. See [GitHub - Ignoring files](https://help.github.com/articles/ignoring-files) for more information about ignoring files.|
+|.rubocop.yml|This file contains the configuration for RuboCop.|
 |.ruby-version|This file contains the default Ruby version.|
 
 Hello, Rails!