Add SMTP upgrade information

Closes #19252

[ci skip]

doc/update/8.8-to-8.9.md

@@ -122,6 +122,19 @@ via [/etc/default/gitlab].
 [Apache templates]: https://gitlab.com/gitlab-org/gitlab-recipes/tree/master/web-server/apache
 [/etc/default/gitlab]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-9-stable/lib/support/init.d/gitlab.default.example#L37
 
+#### SMTP configuration
+
+If you're installing from source and use SMTP to deliver mail, you will need to add the following line
+to config/initializers/smtp_settings.rb:
+
+```ruby
+ActionMailer::Base.delivery_method = :smtp
+```
+
+See [smtp_settings.rb.sample] as an example.
+
+[smtp_settings.rb.sample]: https://gitlab.com/gitlab-org/gitlab-ce/blob/v8.9.0/config/initializers/smtp_settings.rb.sample#L13
+
 #### Init script
 
 Ensure you're still up-to-date with the latest init script changes: