docs: Add note on how to handle off-by-one validation record issue when upgrading to +v2.13.0 (#75)

Author: nickelozz

README.md

@@ -61,6 +61,7 @@ module "acm" {
 ## Notes
 
 * For use in an automated pipeline consider setting the `wait_for_validation = false` to avoid waiting for validation to complete or error after a 45 minute timeout.
+* If you're upgrading to [v2.13.0](https://github.com/terraform-aws-modules/terraform-aws-acm/releases/v2.13.0) or above, you might be subject to [off-by-one validation record issue](https://github.com/terraform-aws-modules/terraform-aws-acm/pull/47#issuecomment-754778599). You can solve this without compromising existing validation records by issuing `terraform state rm <your_module_name>.validation[1]` where `[1]` can be a different index # depending on the number of validation records your module creates (you can check this with `terraform state list module.<your_module_name>.validation`).
 
 <!-- BEGINNING OF PRE-COMMIT-TERRAFORM DOCS HOOK -->
 ## Requirements