Update README.md

Author: mihaelablendea

samples/features/sql-big-data-cluster/deployment/aks/README.md

@@ -45,5 +45,5 @@ When prompted, provide your input for Azure subscription ID, Azure resource grou
 - vm_size - we recommend to use a VM size to accommodate your workload. For an optimal experience while you are validating basic scenarios, we recommend at least 8 vCPUs and 64GB memory across all agent nodes in the cluster. The script uses **Standard_L8s** as default. A default size configuration also uses about 24 disks for persistent volume claims across all components.
 - aks_node_count - this is the number of the worker nodes for the AKS cluster, excluding master node. The script is using a default of 1 agent node. This is the minimum required for this VM size to have enough resources and disks to provision all the necessary persistent volumes.
 - cluster_name - this value is used for both AKS cluster and SQL big data cluster created on top of AKS. Note that the name of the SQL big data cluster is going to be a Kubernetes namespace
-- password - same value is going to be used for all accounts that require user password input: SQL Server master instance SA account, controller user and Knox user
-- controller_username - this is the username for the cluster admin account
+- password - same value is going to be used for all accounts that require user password input: SQL Server master instance account created for the below **username**, controller user and Knox **root** user
+- username - this is the username for the accounts provisioned during deployment for the controller admin account and SQL Server master instance account. Note that **sa** SQL Server account is disabled automatically for you, as a best practice. Username for Knox gateway account is going to be **root**.