Update README.md (#20)

Add update instructions

Author: paulb777

README.md

@@ -3,5 +3,7 @@
 This repo distributes the [gRPC](https://github.com/grpc/grpc) library as a
 binary (xcframework) via Swift Package Manager.
 
+See go/firestore-binary-releasing for update instructions.
+
 > [!WARNING]
 > This repo is not supported as a direct dependency for non-Firebase usage.