Update README to reflect the new AMI build process

Author: scadu

README.md

@@ -114,14 +114,18 @@ AWS_PROFILE="some-profile" make create-stack
 aws-vault exec some-profile -- make create-stack
 ```
 
-If you need to build your own AMI (because you've changed something in the
-`packer` directory), run packer with AWS credentials in your shell environment.
+If you need to build your own AMIs (because you've changed something in the
+`packer` directory), run `make packer` with AWS credentials in your shell environment.
 
-By default, AMIs are built as private (only accessible to the AWS account that created them) for security. You can control AMI visibility and build location using these variables:
+The build process is now two steps:
+1.  **Base AMI:** A base AMI is created with the latest OS updates. This image is cached and only rebuilt when the underlying OS or configuration changes.
+2.  **Final AMI:** The final AMI is built on top of the base AMI, installing the Buildkite agent and other software. This makes final builds faster and more consistent.
 
-- **`AMI_PUBLIC`** - Set to `true` to make AMIs publicly accessible to all AWS users, or `false` (default) for private AMIs
-- **`AMI_USERS`** - Comma-separated list of AWS account IDs that should have access to private AMIs (ignored when `AMI_PUBLIC=true`)
-- **`AWS_REGION`** - AWS region where AMIs should be built (defaults to `us-east-1`)
+By default, AMIs are built as private. You can control AMI visibility and build location using these variables:
+
+- `AMI_PUBLIC` - Set to `true` to make AMIs publicly accessible to all AWS users, or `false` (default) for private AMIs.
+- `AMI_USERS` - A comma-separated list of AWS account IDs that should have access to private AMIs (ignored when `AMI_PUBLIC=true`).
+- `AWS_REGION` - The AWS region where AMIs should be built (defaults to `us-east-1`).
 
 ```bash
 # Build private AMIs (default - recommended for security)
@@ -137,8 +141,7 @@ make packer AMI_USERS="123456789012,987654321098,555666777888"
 make packer AMI_PUBLIC=false AMI_USERS="123456789012,987654321098" AWS_REGION=us-west-2
 ```
 
-This will boot and image three AWS EC2 instances in your account's `us-east-1`
-default VPC (or the region specified by `AWS_REGION`):
+This will create AMIs for the following platforms in your account's `us-east-1` region (or the region specified by `AWS_REGION`):
 
 - Linux (64-bit x86)
 - Linux (64-bit Arm)