Update README.md

Add breadcrumbs for correct role and permission setup.

README.md

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 A [Buildkite plugin](https://buildkite.com/docs/agent/v3/plugins) to login to an ECR repository before running a build step.
 
+This will use standard AWS credentials available [in the environment](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html), or as an [instance role](https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-ec2.html) or task role as available. These must be granted [appropriate permissions](https://docs.aws.amazon.com/AmazonECR/latest/userguide/security_iam_id-based-policy-examples.html) for login to succeed and for push and pull to operate.
+
 ## Example
 
 This will login docker to ECR prior to running your script.