terraform/aws: update dual-subnet example to use peer relay

terraform/aws/aws-ec2-autoscaling-relay-to-private-subnet/README.md

@@ -0,0 +1,54 @@
+# aws-ec2-autoscaling-relay-to-private-subnet
+
+> :information_source: This example is intended for users that have a AWS NAT Gateway that they need to route Internet-bound traffic through. A use case for this is if you have _static IP addresses_ (typically Elastic IPs) that need to use for Internet-bound traffic such as restricting access to GitHub or Snowflake to a custom allow-list of IP addrresses. AWS NAT Gateway is a Hard NAT which forces [DERP connections](https://tailscale.com/kb/1257/connection-types#derp-connections). This example deploys a [Peer Relay](https://tailscale.com/kb/1591/peer-relays) to a Public Subnet to relay traffic to an App Connector in a Private Subnet.
+
+![diagram for aws-ec2-autoscaling-relay-to-private-subnet](./assets/aws-ec2-autoscaling-relay-to-private-subnet.png)
+
+This module creates the following:
+
+- a VPC and related resources including a NAT Gateway, a public subnet, and a private subnet
+- an EC2 Launch Template and Autoscaling Group for a Tailscale Peer Relay in the **_public_** subnet
+- an EC2 Launch Template and Autoscaling Group for a Tailscale App Connector in the **_private_** subnet
+
+## Policy File Example
+
+```json
+{
+    "grants": [
+		//////////////
+		// Peer relays
+		//////////////
+		{
+			"src": ["tag:example-infra"],
+			"dst": ["tag:example-relay"],
+			"app": {
+				"tailscale.com/cap/relay": [],
+			},
+		},
+    ]
+}
+```
+
+## Considerations
+
+- Any advertised routes for the App Connector must still be approved in the Tailscale Admin Console. The code can be updated to use [Auto Approvers for routes](https://tailscale.com/kb/1018/acls/#auto-approvers-for-routes-and-exit-nodes) if this is configured in your ACLs.
+
+## To use
+
+Follow the documentation to configure the Terraform providers:
+
+- [Tailscale](https://registry.terraform.io/providers/tailscale/tailscale/latest/docs)
+- [AWS](https://registry.terraform.io/providers/hashicorp/aws/latest/docs)
+
+### Deploy
+
+```shell
+terraform init
+terraform apply
+```
+
+## To destroy
+
+```shell
+terraform destroy
+```