Update README.md (#1002)

Added note about initConnection needed for listeners to work on iOS.  Fixes issue: #756 (comment)

README.md

@@ -107,7 +107,7 @@ News on Major Releases
 
 Method                                                                                 | Result                    | Description
 ------                                                                                 | ------                    | -----------
-`initConnection()`                                                                     | `Promise<boolean>`        | Init IAP module.<br>On Android this can be called to preload the connection to Play Services.<br>On iOS, it will simply call `canMakePayments` method and return value.
+`initConnection()`                                                                     | `Promise<boolean>`        | Init IAP module.<br>On Android this can be called to preload the connection to Play Services.<br>On iOS, it will simply call `canMakePayments` method and return value which is required for the listeners to work properly.
 `purchaseUpdatedListener((purchase: ProductPurchase) => {})`                           | `EmitterSubscription`　　　| Register a callback that gets called when the store has any updates to purchases that have not yet been finished, consumed or acknowledged. Returns a React Native `EmitterSubscription` on which you can call `.remove()` to stop receiving updates. Register you listener as soon as possible and react to updates at all times.
 `purchaseErrorListener((error: PurchaseError) => {})`                                  | `EmitterSubscription`　　　| Register a callback that gets called when there has been an error with a purchase. Returns a React Native `EmitterSubscription` on which you can call `.remove()` to stop receiving updates.
 `getProducts(skus: string[])`<ul><li>skus: array of Product ID/sku</li></ul>           | `Promise<Product[]>`      | Get a list of products (consumable and non-consumable items, but not subscriptions).<br>Note: With before `iOS 11.2`, this method _will_ also return subscriptions if they are included in your list of SKUs. This is because we cannot differentiate between IAP products and subscriptions prior to `iOS 11.2`.