Add example to documentation for granting AcrPull to the AKS SP

[ninjarobot]

To grant an AKS cluster access to a container registry in the Azure CLI, the AcrPull role is granted on that container registry to the AKS cluster's service principal. This needs to be documented so it's clear how to accomplish this common scenario.

AcrPull role, for reference: https://docs.microsoft.com/en-us/azure/role-based-access-control/built-in-roles#acrpull