updated root readme file.

Author: rahul-infra

README.md

@@ -2,6 +2,10 @@
 # terraform-aws-ecs-deployment
 
 Terraform module to deploy production-ready applications and services on an existing ECS infra.
+This module supports both same-account and cross-account ACM → Route53 validation workflows. For same-account usage, simply map the providers as providers = { aws = aws, aws.cross_account_provider = aws } without any assume role.
+For cross-account setups, you must create an IAM Role in the Route53 Hosted Zone account (Account B) that allows Account A (where ACM and application resources are created) to assume it. This role should grant permissions such as route53:ChangeResourceRecordSets, route53:ListHostedZonesByName, and route53:ListResourceRecordSets, along with a trust policy that permits Account A to assume the role.
+When using cross-account mode, configure an alias provider with assume_role and pass it to the module as:
+providers = { aws = aws, aws.cross_account_provider = aws.cross_account_provider }.
 
 ## Architecture Diagram