Update documentation on usage in README.md (#2)

* Initial commit | Created a module for the mollie payment url over GraphQl

* Added sequences

* Added dependencies

* Moved all logic to new class

* Improved requirements

* Added strict return type and check if mollie transaction doesn't fail

* Added required modules to sequence

* Removed unused import

* Split code and check if order exists

* Removed minimum stability

* Added documentation on the usage of the GraphQl call

README.md

@@ -7,3 +7,19 @@ Add GraphQl support to Mollie Magento 2 module.
 composer require mooore/magento2-module-mollie-graph-ql
 bin/magento setup:upgrade
 ```
+
+## Usage
+```graphql
+mutation($cartId: String!) {
+  placeOrder(input: { cart_id: $cartId }) {
+    order {
+      order_number
+      payment_url
+    }
+  }
+}
+```
+
+The `payment_url` attribute is added to the `CreateOrder` mutation. Using the `payment_url` attribute, the customer can be redirected to iDeal after placing an order.
+
+This modules does not support any payment methods other than Mollie.