Add developer documentation about working with sent emails and previews

doc/development/README.md

@@ -43,6 +43,7 @@
 - [Object state models](object_state_models.md)
 - [Building a package for testing purposes](build_test_package.md)
 - [Manage feature flags](feature_flags.md)
+- [View sent emails or preview mailers](emails.md)
 
 ## Databases
 

doc/development/emails.md

+# Dealing with email in development
+
+## Sent emails
+
+To view rendered emails "sent" in your development instance, visit
+[`/rails/letter_opener`](http://localhost:3000/rails/letter_opener).
+
+## Mailer previews
+
+Rails provides a way to preview our mailer templates in HTML and plaintext using
+dummy data.
+
+The previews live in [`spec/mailers/previews`][previews] and can be viewed at
+[`/rails/mailers`](http://localhost:3000/rails/mailers).
+
+See the [Rails guides] for more info.
+
+[previews]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/spec/mailers/previews
+[Rails guides]: http://guides.rubyonrails.org/action_mailer_basics.html#previewing-emails
+
+---
+
+[Return to Development documentation](README.md)