Merge pull request #15 from sendbird/feature/mininny/update-readme

Update README.md

Author: mininny

README.md

@@ -77,6 +77,8 @@ SendBirdCall.configure("YOUR_APP_ID")
  6. If two iOS devices are available, repeat these steps to install the sample application on both the primary device and the secondary device.
 
 ## Registering push tokens
+In order to make and receive calls, authenticate the user with SendBird server with the `SendBirdCall.authenticate(with:)` method and **register a VoIP push token** to SendBird. You can register a VoIP push token during authentication by either providing it as a parameter in the `authenticate()` method, or after authentication has completed using the `SendBirdCall.registerVoIPPush(token:)` method. VoIP Push Notification will also enable receiving calls even when the app is in the background or terminated state. A valid APNS certificate also needs to be registered on the `SendBird Dashboard` : `Application` → `Settings` → `Notifications` → `Add certificate`.
+
 To handle a native-implementation of receiving incoming calls, you can choose to implement Apple’s [PushKit framework](https://developer.apple.com/documentation/pushkit) and CallKit. This is done by registering device tokens associate with your SendBird Applications. For more information refer to Apple’s [Voice Over IP (VoIP) Best Practices
 ](https://developer.apple.com/library/archive/documentation/Performance/Conceptual/EnergyGuide-iOS/OptimizeVoIP.html)