Add instructions for developers to use embed.js in their web pages

[robbieaverill]

<script src="https://cdn.chec.io/js/embed.js"></script>
<a href="https://checkout.chec.io/wcX9fO"
data-chec-product-id="wcX9fO">Buy now</a>

This is the embed code that the Chec Dashboard gives you for a product. It's using the original version of Embed.js.

The CDN URL for this repository is https://cdn.chec.io/v3/embed.js.

Below is the embed code for a Beam checkout (Commerce Ray):

<script type="text/javascript" src="https://cdn.chec.io/v3/embed.js" async></script>
<a href="javascript:void(0);" data-chec-embed-url="https://dev-sparkling-tomato.commercerays.com">Buy now!</a>

Tasks:

• Ensure this repository works with hosted checkout URLs
• Update the readme of this repository explaining how to use it with both Beam checkout Commerce Rays, and hosted checkout URLs
• Update the product embed code snippet in the Chec Dashboard to use v3 (this version)
• Ensure everything is working correctly