Update i18n guide with local testing setup instructions

Author: deraru

guides/source/i18n.md

@@ -196,8 +196,11 @@ We can also set the locale from the _subdomain_ in a very similar way:
 ```ruby
 # Get locale code from request subdomain (like http://it.application.local:3000)
 # You have to put something like:
-#   127.0.0.1 gr.application.local
+#   127.0.0.1 it.application.local
 # in your /etc/hosts file to try this out locally
+#
+# Additionally, you need to add the following configuration to your config/environments/development.rb:
+#   config.hosts << 'it.application.local:3000'
 def extract_locale_from_subdomain
   parsed_locale = request.subdomains.first
   I18n.available_locales.map(&:to_s).include?(parsed_locale) ? parsed_locale : nil