SPM beta docs (#6312)

SwiftPackageManager.md

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+# Swift Package Manager for Firebase **Beta**
+
+## Introduction
+
+Starting with the 6.31.0 release, Firebase supports installation via [Swift
+Package Manager](https://swift.org/package-manager/) in Beta status.
+
+
+## Limitations
+
+- Requires at least Xcode 12 beta 4.
+- SwiftUI Previews require Xcode 12 beta 5.
+- Analytics requires clients to add `-ObjC` linker option.
+- Analytics is only supported for iOS and cannot be used in apps that support other platforms.
+- Messaging, Performance, Firebase ML, and App Distribution are not initially available.
+- watchOS support is not initially available.
+
+## Installation
+
+If you've previously used CocoaPods, remove them from the project with `pod deintegrate`.
+
+Install Firebase via Swift Package Manager:
+
+<img src="docs/resources/SPMAddPackage.png">
+
+Select the Firebase GitHub repository - `https://github.com/firebase/firebase-ios-sdk.git`:
+
+<img src="docs/resources/SPMChoose.png">
+
+Select the beta branch.
+
+Note: Starting with the 6.31.0 release, the versions are specified
+in a format like 6.31-spm-beta. We won't support standard repository
+versioning until later in the beta or with general availability of the SPM
+distribution.
+
+<img src="docs/resources/SPMSelect.png">
+
+Choose the Firebase product and any additional products that you want installed
+in your app.
+
+<img src="docs/resources/SPMProducts.png">
+
+If you've installed FirebaseAnalytics, Add the `-ObjC` option to `Other Linker Flags`
+in the `Build Settings` tab.
+
+<img src="docs/resources/SPMObjC.png">
+
+## Questions and Issues
+
+Please provide any feedback via a [GitHub
+Issue](https://github.com/firebase/firebase-ios-sdk/issues/new?template=bug_report.md).
+
+See current open Swift Package Manager issues
+[here](https://github.com/firebase/firebase-ios-sdk/labels/Swift%20Package%20Manager).