Created REQUIRED_FIREBASE_OPTIONS_IOS.md (#5347)

* Created REQUIRED_FIREBASE_OPTIONS_IOS.md

Created public documentation that can be linked to from error messages thrown if client-side lenient format checks of Firebase options fail.

b/153238875
https://docs.google.com/document/d/1iuA_LsX650KVlrwS6bN-eVUUQIv_cgKmqMxI7lMGFao/

* Update REQUIRED_FIREBASE_OPTIONS_IOS.md

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FirebaseInstallations/REQUIRED_FIREBASE_OPTIONS_IOS.md

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+# Firebase services require valid Firebase options
+## Firebase Options
+[Firebase options](https://firebase.google.com/docs/reference/ios/firebasecore/api/reference/Classes/FIROptions) are a set of parameters required by services in order to successfully communicate with Firebase server APIs and in order to associate client data with your Firebase project and Firebase application.
+
+Firebase services rely on valid Firebase options being available from the Firebase core SDK [`FIRApp`](https://firebase.google.com/docs/reference/ios/firebasecore/api/reference/Classes/FIRApp), created during Firebase initialization.
+
+Different Firebase services require different Firebase options to function properly, but all Firebase services require the following Firebase options:
+* [**API key**](https://cloud.google.com/docs/authentication/api-keys) - Note: this is **not** an FCM server key, see below. \
+  Example value: `AIzaSyDOCAbC123dEf456GhI789jKl012-MnO`
+* [**Project ID**](https://firebase.google.com/docs/projects/learn-more#project-id) \
+  Example value: `ios-myapp-123`
+* **Application ID**, a.k.a. `GOOGLE_APP_ID`, `mobilesdk_app_id`, "AppId" - Note: this is **not** an Android package name! \
+  Example value: `1:1234567890:ios:321abc456def7890`
+
+## What do I need to know?
+To improve security Firebase SDK updates [on January 14](https://firebase.google.com/support/release-notes/ios#version_6150_-_january_14_2020) and afterwards replaced the Firebase Instance ID service with a dependency on the [Firebase Installations API](https://console.cloud.google.com/apis/library/firebaseinstallations.googleapis.com).
+
+Firebase Installations enforces the existence and validity of mandatory Firebase options API key, Project ID, and Application ID in order to associate client data with your Firebase project.
+
+## Firebase Cloud Messaging (FCM) with Firebase Instance ID (IID)
+If you are reading this message, most likely your application is initializing Firebase without the required set of Firebase options.
+
+Your application may be using an incomplete or erroneous [`GoogleService-Info.plist`](https://firebase.google.com/docs/reference/android/com/google/firebase/FirebaseApp) configuration file; or your app is [programmatically initializing Firebase](https://firebase.google.com/docs/projects/multiprojects) without the full set of required Firebase options.
+
+As a result, Firebase services like Firebase Cloud Messaging (FCM) will malfunction for end-users who installed your app after it was released with the updated Firebase SDKs. Additionally, repeated failing requests to Firebase may slow down the end-user experience of your app.
+
+## What do I need to do?
+To fix malfunctioning Firebase services for your applications, **please take the following steps as soon as possible:**
+1. Update your application by initializing Firebase with a valid API key of your project, a valid Project ID, and a valid Application ID (a.k.a. GOOGLE_APP_ID, mobilesdk_app_id, or simply "App Id").
+    * **Default initialization process using a Firebase config file**: [Download your `GoogleService-Info.plist` config file](https://support.google.com/firebase/answer/7015592) from the Firebase console, then replace the existing file in your app.
+    * **Programmatic initialization using a `FIROptions` object**: [Download your `GoogleService-Info.plist` config file](https://support.google.com/firebase/answer/7015592) from the Firebase console to find your API key, Project ID, and Application ID, then update these values in the `FirebaseOptions` object in your app.
+1. Release a new version of your app to the App Store.
+
+## FCM Server keys
+If your app is using an [FCM Server key](https://firebase.google.com/docs/cloud-messaging/auth-server#authorize-legacy-protocol-send-requests) rather than a Cloud API key, this could cause a security vulnerability in case you are using the same FCM Server key to send push notifications via FCM. \
+In this case, we strongly recommend that you revisit how your server [authenticates send requests to FCM.](https://firebase.google.com/docs/cloud-messaging/auth-server)
+
+Please note that FCM Server Keys (not the same as the Firebase / Cloud API key) must not be included in applications as they can be abused to send push-notifications in the name of your project.
+
+You can [reach out to support](https://firebase.google.com/support/contact?utm_source=email&utm_medium=email&utm_campaign=firebase-installations-api-restrictions-problem) at any time if you have any questions.