SUP-4081 Update README.md for using

Update README.md to ensure users set their agent token before running the AWS CLI command that creates the CFN stack

Author: Mykematt

README.md

@@ -25,7 +25,7 @@ The current release is ![](https://img.shields.io/github/release/buildkite/elast
 
 > Although the stack creates its own VPC by default, Buildkite highly recommends following best practices by setting up a separate development AWS account and using role switching and consolidated billing — see the [Delegate Access Across AWS Accounts tutorial](http://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_cross-account-with-roles.html) for more information.
 
-If you want to use the [AWS CLI](https://aws.amazon.com/cli/), download [`config.json.example`](config.json.example), rename it to `config.json`, and then run the below command:
+If you want to use the [AWS CLI](https://aws.amazon.com/cli/), download [`config.json.example`](config.json.example), rename it to `config.json`, update it with your agent token, then run the below AWS CLI command to create the AWS CloudFormation stack:
 
 ```bash
 aws cloudformation create-stack \