Add default_i18n_subject to the guides

[ci skip]

guides/source/i18n.md

@@ -860,6 +860,24 @@ en:
       subject: "Welcome to Rails Guides!"
 ```
 
+To send params to interpolation use the `default_i18n_subject` method on the mailer.
+
+```ruby
+# user_mailer.rb
+class UserMailer < ActionMailer::Base
+  def welcome(user)
+    mail(to: user.email, subject: default_i18n_subject(user: user.name))
+  end
+end
+```
+
+```yaml
+en:
+  user_mailer:
+    welcome:
+      subject: "%{user}, welcome to Rails Guides!"
+```
+
 ### Overview of Other Built-In Methods that Provide I18n Support
 
 Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers. Here's a brief overview.