If you already have this agreement for your website and you are now launching a mobile app, you need to first consider what kind of personal data you collect via the mobile app.
Then, update your agreement to include the new changes: what you collect from the website and the mobile app. Afterwards, notify users of these changes.
This agreement should be added regardless of the type of platform your business operates on or what kind of industry you are in:
WordPress blogs, or any other platforms: Joomla!, Drupal etc.
Mobile apps: iOS, Android or Windows phone