* Fix - Fix warning shown when using the single-product template.
* Fix - Remove readme.txt.
* Tweak - WC 5.5 compatibility.
* Tweak - WP 5.8 compatibility.
* Fix - Allow One Page Checkout to display when using the SKU attribute of product_page shortcode.
* Fix - Enable One Page Checkout in posts containing the product_page shortcode.
* Tweak - WC 5.5 compatibility.
* Tweak - WP 5.8 compatibility.