Add module versioning and upgrade path

Author: ringanta

README.md

@@ -4,6 +4,23 @@ Terraform module to create an ACM resource that contains domains from multiple R
 ACM validation is using Route53 only.
 This module supports terraform version 0.12 only.
 
+## Module versions
+
+- Module version 1.x is in the `master` branch.
+- Module version 2.x is in the `v2xx` branch.
+
+Module version 2 introduces breaking change. You can't use version 1.x configuration with module 2.x and vice versa.
+To upgrade your existing Terraform project to module version 2, I suggest to provision a new certificate with the same domain name and subject alternatives name. Here is the step would look like:
+
+1. Instantiate `acm-multiple-hosted-zone` version 2 in your existing terraform project.
+1. Provision a new ACM certificate using `acm-multiple-hosted-zone` version 2.
+
+    1. Run `terraform plan -out=tfplan.out`.
+    1. Run `terraform apply tfplan.out`.
+    1. Write down ARN of the newly created ACM certificate.
+
+1. On the consumer side of ACM certificate (For example, ALB), replace old certificate with the new one. This will guarantee a graceful upgrade without downtime.
+
 ## Usage
 
 The `domain_name` and `subject_alternative_names` variables consist of map (object) of string and list of map of string. Each object must consist **zone** and **domain** keys.