Add Read In Environment-Specific Config Values as a Rails til

rails/read-in-environment-specific-config-values.md

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+# Read In Environment-Specific Config Values
+
+The [`config_for`](https://apidock.com/rails/Rails/Application/config_for)
+method allows you to read in a YAML file for the current environment in your
+config files. These YAML files need to be located in the `config` directory.
+
+For instance, let's say you have `config/extra_settings.yml`:
+
+```yml
+development:
+  support_email: support+dev@test.com
+
+production:
+  support_email: support@test.com
+```
+
+You can read that file in as part of your `application.rb` config setup:
+
+```ruby
+# config/application.rb
+module MyApp
+  class Application < Rails::Application
+    extra_settings = config_for(:extra_settings)
+    set_support_email(extra_settings.fetch(:support_email))
+  end
+end
+```
+
+In `development` this call to `config_for` would return a hash of the values
+under `development`. Likewise in `production`.