Add in-app purchase for MacOS

docs/api/in-app-purchase.md

@@ -0,0 +1,36 @@
+# inAppPurchase  _macOS_
+
+Your application should add a listener before to purchase a product. If there are no listener attached to the queue, the payment queue does not synchronize its list of pending transactions with the Apple App Store.
+
+## Methods 
+
+The `inAppPurchase` module has the following methods:
+
+### `inAppPurchase.addTransactionListener(listener)`
+
+* `listener` Function -  Called when transactions are updated by the payment queue.
+  * `payment` Object
+    * `productIdentifier` String
+    * `quantity` Integer
+  * `transaction` Object
+    * `transactionIdentifier` String
+    * `transactionDate` String
+    * `originalTransactionIdentifier` String
+    * `transactionState` String - The transaction sate (`"SKPaymentTransactionStatePurchasing"`, `"SKPaymentTransactionStatePurchased"`, `"SKPaymentTransactionStateFailed"`, `"SKPaymentTransactionStateRestored"`, or `"SKPaymentTransactionStateDeferred"`)
+    * `errorCode` Integer
+    * `errorMessage` String
+   
+
+### `inAppPurchase.purchaseProduct(productID, quantity, callback)`
+* `productID` String - The id of the product to purchase. (the id of `com.example.app.product1` is `product1`).
+* `quantity` Integer (optional) - The number of items the user wants to purchase.
+* `callback` Function (optional) - The callback called when the payment is added to the PaymentQueue. (You should add a listener with `inAppPurchase.addTransactionsListener` to get the transaction status).
+  * `isProductValid` Boolean - Determine if the product is valid and added to the payment queue.
+
+### `inAppPurchase.canMakePayments()`
+
+Returns `true` if the user can make a payment and `false` otherwise.
+
+### `inAppPurchase.getReceiptURL()`
+
+Returns `String`, the path to the receipt.