magento#589: Add a tutorial for the checkout workflow

Author: atwixfirster

guides/v2.3/graphql/tutorials/checkout/checkout-add-product-to-cart.md

@@ -11,23 +11,30 @@ contributor_name: Atwix
 contributor_link: https://www.atwix.com/
 ---
 
+GraphQL supports [7 types of product]({{ page.baseurl }}/graphql/reference/product-interface-implementations.html) which can be added into shopping cart:
+ - bundle product
+ - configurable product
+ - downloadable product
+ - grouped product
+ - simple product
+ - virtual product
+ - gift card product (available for commerce version)
+
 **Request**
+The following query adds a simple product into shopping cart.
 ```text
-mutation addSimpleProductsToCart(
-  $cart_id: String!
-  $qty: Float!
-  $sku: String!
-) {
+mutation {  
   addSimpleProductsToCart(
     input: {
-      cart_id: $cart_id
-      cartItems: {
-        customizable_options: []
-        data: {
-          qty: $qty
-          sku: $sku
+      cart_id: "{{ CART_ID }}"
+      cartItems: [
+        {
+          data: {
+            qty: 1
+            sku: "simple-product"
+          }
         }
-      }
+      ]
     }
   ) {
     cart {
@@ -44,6 +51,32 @@ mutation addSimpleProductsToCart(
 }
 ```
 
+where 
+`{{ CART_ID }}` - shopping cart unique ID from [Step 2. Create empty cart]({{ page.baseurl }}/graphql/tutorials/checkout/checkout-add-product-to-cart.html).
+
 **Response**
 ```json
-```
+{
+  "data": {
+    "addSimpleProductsToCart": {
+      "cart": {
+        "items": [
+          {
+            "id": "508",
+            "product": {
+              "sku": "simple-product",
+              "stock_status": "IN_STOCK"
+            },
+            "qty": 1
+          }
+        ]
+      }
+    }
+  }
+}
+```
+
+{:.bs-callout .bs-callout-info}
+Send customer's authorization token in the `Authorization` parameter of the header if you add product into shopping cart as a registered customer. See ["Get customer authorization token"]({{ page.baseurl }}/graphql/get-customer-authorization-token.html) to get more details.
+
+Use `updateCartItems` mutation query to update shopping cart items and `removeItemFromCart` to remove product from the shopping cart.