Update updating-agent docs

Author: keithduncan

docs/updating-agent.md

@@ -1,17 +1,31 @@
 # Updating the Agent
 
+The `buildkite-agent` is built in to the AMIs by the Packer build.
+
+Once you have [released](https://github.com/buildkite/agent/blob/master/RELEASE.md) an updated
+version of the agent, you can incorporate it into the Elastic CI Stack
+for AWS template.
+
+See https://github.com/buildkite/elastic-ci-stack-for-aws/pull/935 for an
+example of updating the buildkite-agent.
+
+1. Create a new branch
+1. Update and commit a change to the Packer install scripts for [Linux](#linux) and [Windows](#windows) for the new version
+1. Push your branch and open a pull request
+1. Wait for CI to go green
+1. Merge
+
 ## Linux
 
-Packer installs the `buildkite-agent` using the [`packer/linux/scripts/install-buildkite-agent.sh`](packer/linux/scripts/install-buildkite-agent.sh)
-script. Update the `AGENT_VERSION` variable in this file to
-change which version is installed.
+Update the `AGENT_VERSION` variable in [`packer/linux/scripts/install-buildkite-agent.sh`](packer/linux/scripts/install-buildkite-agent.sh)
+to change which version is installed during the Packer build for the Linux AMI.
 
 The agent binary is downloaded from download.buildkite.com.
 
 ## Windows
 
-Packer installs the `buildkite-agent` using the [`packer/windows/scripts/install-buildkite-agent.ps1`](packer/windows/scripts/install-buildkite-agent.ps1)
-script. Update the `AGENT_VERSION` variable in this file to
-change which version is installed.
+Update the `AGENT_VERSION` variable in [`packer/windows/scripts/install-buildkite-agent.ps1`](packer/windows/scripts/install-buildkite-agent.ps1)
+to change which version is installed during the Packer build for the Windows
+AMI.
 
 The agent binary is downloaded from download.buildkite.com.