Issue #2974889 by john_a: add a Drupal message to marketing_cloud_example hook help()

modules/marketing_cloud_example/marketing_cloud_example.module

 <?php
+
 /**
  * @file
  * Adds example sub-module for demonstrating how to use Marketing Cloud.
@@ -15,6 +16,8 @@ function marketing_cloud_example_help($route_name, RouteMatchInterface $route_ma
 
       $text = file_get_contents(dirname(__FILE__) . '/README.md');
       if (!\Drupal::moduleHandler()->moduleExists('markdown')) {
+        drupal_set_message('To view this page correctly, please install and
+         enable the markdown module.');
         return '<pre>' . $text . '</pre>';
       }
       else {