docs(readme): update readme

README.md

@@ -1,2 +1,27 @@
 # RxStoreKit
+
 Reactive extensions of StoreKit
+
+## How to use
+
+### Request SKProduct
+
+Create observable sequence of responses for SKProductRequest and subscribe it
+
+``` swift
+SKProduct.rx.request(with: ["xxxxx"])
+    .subscribe(onNext: { response in
+        let products = response.products
+    })
+    .dispose()
+```
+
+### Subscribe update payment transactons
+
+```swift
+ SKPaymentQueue.default().rx.updatedTransaction
+    .subscribe(onNext: { transaction in
+        // Do what you want
+    })
+    .dispose()
+```