Merge pull request #21996 from tak1n/4-2-stable

Add config_for custom configuration example for rails 4.2

guides/source/configuring.md

@@ -1043,3 +1043,30 @@ These configuration points are then available through the configuration object:
   Rails.configuration.x.super_debugger              # => true
   Rails.configuration.x.super_debugger.not_set      # => nil
   ```
+
+You can also use Rails::Application.config_for to load whole configuration files:
+
+  ```ruby
+  # config/payment.yml:
+  production:
+    environment: production
+    merchant_id: production_merchant_id
+    public_key:  production_public_key
+    private_key: production_private_key
+  development:
+    environment: sandbox
+    merchant_id: development_merchant_id
+    public_key:  development_public_key
+    private_key: development_private_key
+
+  # config/application.rb
+  module MyApp
+    class Application < Rails::Application
+      config.x.payment = Rails.application.config_for(:payment)
+    end
+  end
+  ```
+
+  ```ruby
+  Rails.configuration.x.payment.merchant_id # => production_merchant_id or development_merchant_id
+  ```