Add Readme and link to Firebase Data Connect (#15220)

Author: aashishpatil-g

FirebaseDataConnect/README.md

@@ -0,0 +1,5 @@
+# Firebase Data Connect Swift SDK
+
+**Connect your Swift & SwiftUI apps directly to a managed Google CloudSQL (PostgreSQL) database.**
+
+The SDK resides in a separate [git repo](https://github.com/firebase/data-connect-ios-sdk)