Update documentation to customize the application config

docs/en/customization/application.md

@@ -0,0 +1,25 @@
+# Customizing application configuration
+
+## Overwriting application.rb
+
+If you need to extend or modify the `config/application.rb` file, you can do so using the `config/application_custom.rb` file. For example, if you'd like to change the application timezone to the Canary Islands, just add:
+
+```ruby
+module Consul
+  class Application < Rails::Application
+    config.time_zone = "Atlantic/Canary"
+  end
+end
+```
+
+In this particular case, note that the application time zone can alternatively be modified by editing the `config/secrets.yml` file.
+
+Remember that in order to apply these changes you'll need to restart the application.
+
+## Overwriting environment configuration
+
+If you'd like to customize the development, test, staging, preproduction or production environments, you can do so by editing the files under `config/environments/custom/`.
+
+Note that changes in `config/environments/custom/production.rb` are also applied to the staging and preproduction environments, and changes in `config/environments/custom/staging.rb` are also applied to the preproduction environment.
+
+And remember that in order to apply these changes you'll need to restart the application.