Update cart.md (#7700)

* Update cart.md

Add information on how to add extra data to the /add-item request

* Update src/StoreApi/docs/cart.md

Co-authored-by: Niels Lange <[email protected]>

* Update src/StoreApi/docs/cart.md

Co-authored-by: Niels Lange <[email protected]>

Author: ralucaStan

src/StoreApi/docs/cart.md

@@ -332,6 +332,8 @@ curl --header "Nonce: 12345" --request POST https://example-store.com/wp-json/wc
 
 Returns the full [Cart Response](#cart-response) on success, or an [Error Response](#error-response) on failure.
 
+If you want to add supplemental cart item data before it is passed into `CartController::add_to_cart` use the [`woocommerce_store_api_add_to_cart_data`](https://github.com/woocommerce/woocommerce-blocks/blob/4d1c295a2bace9a4f6397cfd5469db31083d477a/docs/third-party-developers/extensibility/hooks/filters.md#woocommerce_store_api_add_to_cart_data) filter.
+
 ## Remove Item
 
 Remove an item from the cart and return the full cart response, or an error.