Update SwiftPackageManager.md with instructions for Crashlytics SPM (#7229)

Author: elenadoty

SwiftPackageManager.md

@@ -51,6 +51,10 @@ https://github.com/firebase/firebase-ios-sdk/issues/6472#issuecomment-694449182.
 
 <img src="docs/resources/SPMObjC.png">
 
+If you're using FirebaseCrashlytics, use
+`${BUILD_DIR}SourcePackages/checkouts/firebase-ios-sdk/Crashlytics/run`
+as the run script that allows Xcode to upload your project's dSYM files.
+
 ### Alternatively, add Firebase to a `Package.swift` manifest
 
 To integrate via a `Package.swift` manifest instead of Xcode, you can add