Document workaround for Xcode 12 GM SPM binary device install bug (#6499)

Author: paulb777

SwiftPackageManager.md

@@ -44,6 +44,10 @@ Choose the Firebase products that you want installed in your app.
 If you've installed FirebaseAnalytics, Add the `-ObjC` option to `Other Linker Flags`
 in the `Build Settings` tab.
 
+If you're using FirebaseAnalytics, Xcode 12.0, and have an issue with
+device installation or archive uploading, see the workaround at
+https://github.com/firebase/firebase-ios-sdk/issues/6472#issuecomment-694449182.
+
 <img src="docs/resources/SPMObjC.png">
 
 ### Alternatively, add Firebase to a `Package.swift` manifest