Update README.md

README.md

@@ -59,7 +59,7 @@ Nb. the API key is *not* your Mailgun password (nor your domain's SMTP
 password). You can find it on the page for your domain on the Mailgun admin
 interface.
 
-Once you've got it, visit the settings page at:
+**CiviCRM 5.8+ users can** visit the settings page at:
 
 - Drupal 7: `https://example.com/civicrm/mailgunny/settings`
 - Wordpress: `https://example.com/?page=CiviCRM&q=civicrm/mailgunny/settings`
@@ -67,6 +67,12 @@ Once you've got it, visit the settings page at:
 
 Put it in the box and press Save.
 
+**CiviCRM 5.0 - 5.7 users** must instead define a constant in their `civicrm.settings.php` file like this:
+
+```php
+define('MAILGUN_API_KEY', 'xxxxxxxxxxxxxxxxx');
+```
+
 Right, you're all set.