Add push credential docs (#143)

Author: idelgado

Images/update_push_credential.png

@@ -4,6 +4,7 @@
 * [Quickstart](#quickstart) - Run the quickstart app
 * [Access Tokens](#access-tokens) - Using access tokens
 * [Managing Audio Interruptions](#managing-audio-interruptions) - Managing audio interruptions
+* [Managing Push Credentials](#managing-push-credentials) - Managing push credentials
 * [More Documentation](#more-documentation) - More documentation related to the Voice iOS SDK
 * [Issues and Support](#issues-and-support) - Filing issues and general support
 
@@ -281,6 +282,30 @@ CXTransaction *transaction = [[CXTransaction alloc] initWithAction:setHeldCallAc
 }];
 ```
 
+## Managing Push Credentials
+
+A push credential is a record for a push notification channel, for iOS this push credential is a push notification channel record for APNS VoIP. Push credentials are managed in the console under [Mobile Push Credentials](https://www.twilio.com/console/voice/sdks/credentials).
+
+Whenever a registration is performed via `TwilioVoice.registerWithAccessToken:deviceToken:completion` in the iOS SDK the `identity` and the `Push Credential SID` provided in the JWT based access token, along with the `device token` are used as a unique address to send APNS VoIP push notifications to this application instance whenever a call is made to reach that `identity`. Using `TwilioVoice.unregisterWithAccessToken:deviceToken:completion` removes the association for that `identity`.
+
+### Updating a Push Credential
+
+If you need to change or update your credentials provided by Apple you can do so by selecting the Push Credential in the [console](https://www.twilio.com/console/voice/sdks/credentials) and adding your new `certificate` and `private key` in the text box provided on the Push Credential page shown below:
+
+<kbd><img height="667px" src="Images/update_push_credential.png"/></kbd>
+
+### Deleting a Push Credential
+
+We **do not recommend that you delete a Push Credential** unless the application that it was created for is no longer being used.
+
+If **your APNS VoIP certificate is expiring soon or has expired you should not delete your Push Credential**, instead you should update the Push Credential by following the `Updating a Push Credential`  section.
+
+When a Push Credential is deleted **any associated registrations made with this Push Credential will be deleted**. Future attempts to reach an `identity` that was registered using the Push Credential SID of this deleted push credential **will fail**.
+
+If you are certain you want to delete a Push Credential you can click on `Delete this Credential` on the [console](https://www.twilio.com/console/voice/sdks/credentials) page of the selected Push Credential.
+
+Please ensure that after deleting the Push Credential you remove or replace the Push Credential SID when generating new access tokens.
+
 ## More Documentation
 You can find more documentation on getting started as well as our latest AppleDoc below: