Add docs on updating buildkite-agent-scaler

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch keithduncan/add-updating-docs
# Your branch is up to date with 'origin/keithduncan/add-updating-docs'.
#
# Changes to be committed:
#	new file:   docs/updating-buildkite-agent-scaler.md
#

Author: keithduncan

docs/updating-buildkite-agent-scaler.md

@@ -0,0 +1,25 @@
+# Updating buildkite-agent-scaler
+
+The [buildkite-agent-scaler](https://buildkite.com/buildkite/buildkite-agent-scaler)
+is brought in via the AWS Serverless Application Repository.
+
+Once you have [released](https://github.com/buildkite/buildkite-agent-scaler/blob/master/RELEASE.md) an updated
+version, you can incorporate it into the Elastic CI Stack
+for AWS template.
+
+1. Create a new branch
+1. In the [`templates/aws-stack.yml`](templates/aws-stack.yml)update the `Autoscaling` resource’s `SemanticVersion` property to the newly released version.
+1. Push your branch and open a pull request
+1. Wait for CI to pass
+1. If needed for testing, create a stack with the branch’s published template to verify functionality
+	1. In the Buildkite build, get the template URL from the build annotation
+	1. In the AWS Console, launch CloudFormation
+	1. Create a new stack, select new resources
+	1. Supply the template URL from the build annotation for the Amazon S3 URL field
+	1. Supply a Buildkite Agent token in the `BuildkiteAgentToken` 
+	or `BuildkiteAgentTokenParameterStorePath` parameters
+	1. Supply a queue in the `BuildkiteQueue` parameter
+	1. Create the stack and wait for it to complete
+	1. Verify that instances are booted in response to Buildkite jobs on your queue
+	1. Delete the stack
+1. Merge your pull request