Add link to Rack website

Resemble links to Bundler and Yarn,
based on feedback by @kamipo

[ci skip]

guides/source/getting_started.md

@@ -172,7 +172,7 @@ of the files and folders that Rails created by default:
 |app/|Contains the controllers, models, views, helpers, mailers, channels, jobs and assets for your application. You'll focus on this folder for the remainder of this guide.|
 |bin/|Contains the rails script that starts your app and can contain other scripts you use to setup, update, deploy or run your application.|
 |config/|Configure your application's routes, database, and more. This is covered in more detail in [Configuring Rails Applications](configuring.html).|
-|config.ru|Rack configuration for Rack based servers used to start the application.|
+|config.ru|Rack configuration for Rack based servers used to start the application. For more information about Rack, see the [Rack website](https://rack.github.io/).|
 |db/|Contains your current database schema, as well as the database migrations.|
 |Gemfile<br>Gemfile.lock|These files allow you to specify what gem dependencies are needed for your Rails application. These files are used by the Bundler gem. For more information about Bundler, see the [Bundler website](https://bundler.io).|
 |lib/|Extended modules for your application.|