update example config

config.sample.yml

@@ -1,4 +1,8 @@
 port: 4080
 mailer:
+  service: 'mailgun'
+  url: 'https://api.mailgun.net/v3/example.com/messages'
   api_key: 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
+  # Public Key at https://app.mailgun.com/app/account/security
+  api_public_key: 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
   from: 'mailer@example.com'