docs: update README.md to include upgrade info (#25)

Author: jasonwalsh

README.md

@@ -2,6 +2,10 @@
 
 > A Terraform module for creating Amazon [ECS Task Definitions](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definitions.html)
 
+## NOTICE
+
+**THIS MODULE IS NOT COMPATIBLE WITH VERSIONS OF TERRAFORM LESS THAN v0.12.x. PLEASE REFER TO THE OFFICIAL [DOCUMENTATION](https://www.terraform.io/upgrade-guides/0-12.html) FOR UPGRADING TO THE LATEST VERSION OF TERRAFORM.**
+
 ## Contents
 
 - [Motivation](#motivation)