Update README.md

Author: morgante

README.md

@@ -1,5 +1,7 @@
 # NAT Gateway Terraform Module
 
+## NOTE: This module is no longer maintained. Please use [Cloud NAT](https://github.com/terraform-google-modules/terraform-google-cloud-nat) instead.
+
 Modular NAT Gateway on Google Compute Engine for Terraform.
 
 <a href="https://concourse-tf.gcp.solutions/teams/main/pipelines/tf-nat-gw-regression" target="_blank">