docs: add guest cart/mine cart edit item endpoints

Author: ahinkle

README.md

@@ -131,6 +131,13 @@ Add/update the specified cart item with a customer token. Must have a store code
 Magento::api('cartItems')->addItem($cartId, $sku, $quantity);
 ```
 
+`put` - `/V1/carts/mine/items/{itemId}`
+
+Update the specified cart item with a customer token. Must have a store code.
+```php
+Magento::api('cartItems')->editItem($itemId, $body = []);
+```
+
 Remove the specified cart item with a customer token. Must have a store code.
 ```php
 Magento::api('cartItems')->removeItem($itemId);
@@ -220,6 +227,13 @@ Add/update the specified cart item.
 Magento::api('guestCarts')->addItem($cartId, $sku, $quantity);
 ```
 
+`put` - `/V1/guest-carts/{cartId}/items/{itemId}`
+
+Update the specified cart item.
+```php
+Magento::api('guestCarts')->editItem($cartId, $itemId, $body = []);
+```
+
 Remove the specified cart item.
 ```php
 Magento::api('guestCarts')->removeItem($cartId, $itemId);