Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/modules/contact/config/install/contact.form.personal.yml

@@ -6,3 +6,5 @@ label: 'Personal contact form'
 recipients: {  }
 reply: ''
 weight: 0
+message: 'Your message has been sent.'
+redirect: ''

core/modules/contact/config/schema/contact.schema.yml

@@ -22,6 +22,12 @@ contact.form.*:
     weight:
       type: integer
       label: 'Weight'
+    message:
+      type: label
+      label: 'Message displayed to user on submission'
+    redirect:
+      type: path
+      label: 'Redirect path on submission'
 
 contact.settings:
   type: config_object