Edit readme to make it clearer

[aurian]

In file https://github.com/pantheon-systems/quicksilver-examples/blob/master/slack_notification/README.md, change "Add, and update as needed, the example slack_notification.php script to the private directory in the root of your site's codebase, that is under version control. Note this is a different private directory than where the secrets.json is stored."
For: "Add, and update as needed, the example slack_notification.php script to the private directory in the root of your site's codebase, that is under version control. It should be located under code/private/scripts/slack_notification.php."