docs: add removeItem()

Author: ahinkle

README.md

@@ -110,6 +110,11 @@ Add/update the specified cart item with a customer token. Must have a store code
 Magento::api('cartItems')->addItem($cartId, $sku, $quantity);
 ```
 
+Remove the specified cart item with a customer token. Must have a store code.
+```php
+Magento::api('cartItems')->removeItem($itemId);
+```
+
 #### Cart Totals (quoteCartTotalRepositoryV1)
 
 `/V1/carts/mine/totals`
@@ -180,6 +185,11 @@ Add/update the specified cart item.
 Magento::api('guestCarts')->addItem($cartId, $sku, $quantity);
 ```
 
+Remove the specified cart item.
+```php
+Magento::api('guestCarts')->removeItem($cartId, $itemId);
+```
+
 `/V1/guest-carts/{cartId}/estimate-shipping-methods`
 
 Estimate shipping by address and return list of available shipping methods.