Update Firebase Phone Auth docs.

Author: yramanchuk

FirebasePhoneAuthUI/README.md

@@ -7,6 +7,7 @@ You can use Firebase Phone Authentication to sign in a user by sending an SMS me
 1. [Installation](#installation)
 1. [Integration](#using-firebasephoneui-for-authentication)
 1. [Customization](#customizing)
+1. [Integration cheat sheet](#integration-cheat-sheet)
 
 ## Installation
 ### Importing FirebaseUI Phone Auth components
@@ -69,4 +70,39 @@ FUIPhoneAuth *phoneProvider = [FUIAuth defaultAuthUI].providers.firstObject;
 ```
 
 ## Customizing
-Customizing of Phone Auth is planned to be implemented in 2017 Q4
+Customizing of Phone Auth is planned to be implemented in 2017 Q4
+
+## Integration cheat sheet
+Here you can find steps things that need to be checked in case of any issues with Firebase Phone Auth integration problems.
+
+In case  need to handle push notifications yourself:
+
+1. Add `APNS Key` or `APNS cert` to Firebase console project.
+<br>If `APNS cert` is used than check that you uploaded certificate with the same `bundleID` as Firebase iOS app `bundleID`.
+1. In the Xcode `Project settings` -> `Capabilities` enable `Push Notifications`
+1. In the project `Info.plist` set to `NO` value of `FirebaseAppDelegateProxyEnabled` (add this key if needed)
+1. In the `AppDelegate` `didRegisterForRemoteNotificationsWithDeviceToken` call `[FUIAuth.defaultAuthUI.auth setAPNSToken:deviceToken]`
+<br>In this case The type of the token (production or sandbox) will be attempted to be automatically detected. There is other method to set it manually.
+1. In the `AppDelegate` `application:didReceiveRemoteNotification:fetchCompletionHandler:` call `[FUIAuth.defaultAuthUI.auth canHandleNotification:userInfo]`
+1. In the `AppDelegate` `application:didFinishLaunchingWithOptions:` call `[FIRApp configure]`
+1. In the `AppDelegate` `application:openURL:options:` return `[FUIAuth.defaultAuthUI handleOpenURL:url sourceApplication:sourceApplication]` 
+1. Add `REVERSED_CLIENT_ID` as URL scheme to `Project settings`
+1. Add `GoogleService-Info.plist` to your project
+1. In you controller call:
+```objective-c
+  [FUIAuth defaultAuthUI].delegate = self; // delegate should be retained by you!
+  FUIPhoneAuth *phoneProvider = [[FUIPhoneAuth alloc] initWithAuthUI:[FUIAuth defaultAuthUI]];
+  [FUIAuth defaultAuthUI].providers = \@[phoneProvider];
+```
+11\. To start Phone Auth, please call:
+```objective-c
+  FUIPhoneAuth *phoneProvider = [FUIAuth defaultAuthUI].providers.firstObject;
+  [phoneProvider signInWithPresentingViewController:self phoneNumber:nil];
+```
+You can skip all errors, FirbaseUI Phone Auth will display all error messages for you.
+<br>You may need only to handle error code `FUIAuthErrorCodeUserCancelledSignIn`.
+
+If you don't need to handle APNS yourself, than don't do steps 3, 4, 5.
+
+Please notice that you can use either APNS key OR APNS certificate.
+<br>One APNS Key can be used for all your iOS app from the same Apple Developer account.