Cherry-pick AppleSilicon instructions. (#7081)

Author: ryanwilson

AppleSilicon.md

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+# Apple Silicon Simulator Support
+
+If you're using Swift Package Manager or the zip file, everything should work for you
+using Xcode 12.0 or above. See the instructions below for CocoaPods and Carthage.
+
+## CocoaPods
+
+All source-based Firebase CocoaPods work as expected. Binary pods require CocoaPopds version 1.10.0
+and are listed below with the current status.
+
+### Analytics + GoogleAppMeasurement
+
+As of Firebase 7.2.0, FirebaseAnalytics and GoogleAppMeasurement provide a separate distribution of
+an XCFramework in order to work around an issue with CocoaPods and static XCFrameworks.
+
+**IMPORTANT**: After adding the CocoaPod, you must manually add `-ObjC` to your `Other Linker Flags`
+in order to avoid a runtime crash.
+
+This is a temporary workaround while Analytics is affected by the CocoaPods bug.
+
+When specifying which version of Firebase you'd like in your Podfile, append `-M1` to the version.
+See the following examples:
+
+```
+# Explicitly require the special `M1` tagged Firebase version, locked to the major version.
+pod 'Firebase/Analytics', '~> 7.2-M1'
+
+# Do the same for any other Firebase pod used.
+pod 'Firebase/Database', '~> 7.2-M1'
+
+# You can also lock on the minor or patch versions like so:
+pod 'Firebase/Analytics, '~> 7.2.0-M1'
+pod 'Firebase/Analytics, '7.2.0-M1'
+```
+
+Remember: now you need to manually add `-ObjC` to avoid a runtime crash. This CocoaPods issue has
+been fixed in
+[CocoaPods/CocoaPods#10234](https://github.com/CocoaPods/CocoaPods/pull/10234) and is merged for
+the upcoming CocoaPods 1.10.1 patch release.
+
+### Performance
+
+As of Firebase 7.2.0, FirebasePerformance uses an XCFramework for distribution that works with
+Apple silicon.
+
+### FirebaseML
+
+FirebaseML does not yet work with the simulator on Apple silicon Macs.
+
+## Carthage
+
+XCFrameworks are required to include the arm64 slice for iOS devices and the macOS simulator for
+Macs running on Apple silicon. Unfortunately, Carthage does not support XCFrameworks yet which
+prevents us from being able to include support. See
+[Carthage/Carthage#2799](https://github.com/Carthage/Carthage/issues/2799) for progress.
+