add example in usage.md

Author: mmoutama09

docs/usage.md

@@ -144,6 +144,37 @@ module "runner" {
 }
 ```
 
+### Scenario: Use of Docker autoscaler
+
+As docker machine is no longer maintained by docker, gitlab recently developed docker_autoscaler to replace docker machine (still in beta). An option is available to test it out. It reuses the arguments int he docker-machine block arguments and create an autoscaling groupthat will be supplied to docker-autoscaler.
+
+#### Configuration example
+
+```hcl
+module "runner" {
+  # https://registry.terraform.io/modules/cattle-ops/gitlab-runner/aws/
+  source  = "cattle-ops/gitlab-runner/aws"
+
+  vpc_id    = module.vpc.vpc_id
+  subnet_id = element(module.vpc.private_subnets, 0)
+
+  runner_gitlab = {
+    url = "https://gitlab.com"
+    preregistered_runner_token_ssm_parameter_name = "my-gitlab-runner-token-ssm-parameter-name"
+ }
+
+  runner_worker = {
+    type = "docker_autoscaler"
+  }
+
+  runner_worker_docker_machine_instance = {
+    subnet_ids = vpc.private_subnet_ids
+    types      = ["t3a.medium", "t3.medium"]
+  }
+
+}
+```
+
 ## Examples
 
 A few [examples](https://github.com/cattle-ops/terraform-aws-gitlab-runner/tree/main/examples/) are provided. Use the