Add notice/banner to readme.txt about new shipping extension

[kloon]

Let's add a banner or notice to the readme.txt file to show at the top of the listing on wp.org to indicate that shipping functionality has moved to a new dedicated plugin with a link to https://wordpress.org/plugins/woocommerce-shipping/

Preferably a banner that has a background if possible.

Attention: Shipping features have moved to a new dedicated plugin. Download the new plugin.