Document rails new <app> storage folder

[ci skip] Active Storage now adds a storage folder to newly generated
rails applications.

guides/source/getting_started.md

@@ -179,6 +179,7 @@ of the files and folders that Rails created by default:
 |public/|The only folder seen by the world as-is. Contains static files and compiled assets.|
 |Rakefile|This file locates and loads tasks that can be run from the command line. The task definitions are defined throughout the components of Rails. Rather than changing `Rakefile`, you should add your own tasks by adding files to the `lib/tasks` directory of your application.|
 |README.md|This is a brief instruction manual for your application. You should edit this file to tell others what your application does, how to set it up, and so on.|
+|storage/|Active Storage files for Disk Service. This is covered in [Active Storage Overview](active_storage_overview.html).|
 |test/|Unit tests, fixtures, and other test apparatus. These are covered in [Testing Rails Applications](testing.html).|
 |tmp/|Temporary files (like cache and pid files).|
 |vendor/|A place for all third-party code. In a typical Rails application this includes vendored gems.|