show talkable integration steps

**Plastc/Talkable Instructions**

1. add `<script src="//d2jjzw81hqbuqv.cloudfront.net/integration/clients/plastc.min.js"></script>` on the page with the checkout

1. add the Talkable code in `src/js/app.js` in the onConfirmation method

1. in your website css, you need to have this snippet:

```css
#talkable-container-pp iframe {
  z-index: 2000000002 !important;
}
```