Document AWS CNI requirements

[oblogic7]

The documentation for this operator is lacking some important details about the aws-cni configuration that are required.

You must be using the AWS VPC CNI plugin. This is the default for AWS EKS, so you probably don't need to change anything here.

The above statement fails to mention that the CNI must be configured to enable security groups for pods. This is because the operator relies on the presence of the pod-eni annotation for ENI selection. This annotation is not configured on the default aws-cni configuration.