docs: Add note for upgrade Terraform Provider

Author: npalm

Diff for: README.md

@@ -4,12 +4,8 @@
 
 This [Terraform](https://www.terraform.io/) module creates the required infrastructure needed to host [GitHub Actions](https://github.com/features/actions) self-hosted, auto-scaling runners on [AWS spot instances](https://aws.amazon.com/ec2/spot/). It provides the required logic to handle the life cycle for scaling up and down using a set of AWS Lambda functions. Runners are scaled down to zero to avoid costs when no workflows are active.
 
-> NEW: Ephemeral runners available as beta feature.
-
-> NEW: Windows runners are available.
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-> NEW: Examples for custom AMI are available.
-
+> BREAKING CHANGE: The module is upgraded to Terraform AWS provider 4.x. All new development will only support the new AWS Terraform provider. We keep a branch `terraform-aws-provider-3` to witch we welcome backports to AWS Terraform 3.x provider. Besides reviewing PR's we will do not any active checking on maintance on this branch. We strongly advise to update your deployment to the new provider version. For more details about upgrading see the [upgrade guide](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/guides/version-4-upgrade).
+ 
 - [Motivation](#motivation)
 - [Overview](#overview)
   - [Major configuration options.](#major-configuration-options)