Add smtp_settings sample

config/initializers/smtp_settings.rb.sample

+# To enable smtp email delivery for your GitLab instance do next: 
+# 1. Change config/environments/production.rb to use smtp
+#    config.action_mailer.delivery_method = :smtp
+# 2. Rename this file to smpt_settings.rb
+# 3. Edit settings inside this file
+# 4. Restart GitLab instance
+#
+if Gitlab::Application.config.action_mailer.delivery_method == :smtp
+  ActionMailer::Base.smtp_settings = {
+    address: "email.server.com",
+    port: 456,
+    user_name: "smtp",
+    password: "123456",
+    domain: "gitlab.company.com",
+    authentication: :login,
+    enable_starttls_auto: true
+  }
+end