You add a rule action to show a notification for a selected email.

The rule notification action will insert text before and/or after the email subject or replace it completely with your own text. When you add the notify action, you can type your own text into the text field. If you want to include the current subject text into the new subject, include this text token:
<Subject>

eg. If you receive an email with the subject "New product" and want to insert "Great " before the subject, then you can type this text in the action:
Great <Subject>
This will show a notification with the text "Great New product"