Update aks-naming-rules and 1804 nodepool create

Author: palma21

articles/aks/cluster-configuration.md

@@ -44,6 +44,8 @@ When the status shows as registered, refresh the registration of the `Microsoft.
 az provider register --namespace Microsoft.ContainerService
 ```
 
+### New clusters
+
 Configure the cluster to use Ubuntu 18.04 when the cluster is created. Use the `--aks-custom-headers` flag to set the Ubuntu 18.04 as the default OS.
 
 ```azure-cli
@@ -52,6 +54,17 @@ az aks create --name myAKSCluster --resource-group myResourceGroup --aks-custom-
 
 If you want to create a regular Ubuntu 16.04 cluster, you can do so by omitting the custom `--aks-custom-headers` tag.
 
+### Existing clusters
+
+Configure a new nodepool to use Ubuntu 18.04. Use the `--aks-custom-headers` flag to set the Ubuntu 18.04 as the default OS for that nodepool.
+
+```azure-cli
+az aks nodepool add --name ubuntu1804 --cluster-name myAKSCluster --resource-group myResourceGroup --aks-custom-headers CustomizedUbuntu=aks-ubuntu-1804
+```
+
+If you want to create a regular Ubuntu 16.04 nodepools, you can do so by omitting the custom `--aks-custom-headers` tag.
+
+
 ## Custom resource group name
 
 When you deploy an Azure Kubernetes Service cluster in Azure, a second resource group gets created for the worker nodes. By default, AKS will name the node resource group `MC_resourcegroupname_clustername_location`, but you can also provide your own name.

articles/aks/troubleshooting.md

@@ -113,8 +113,9 @@ Follow the *Before you begin* steps in the appropriate doc to correctly create a
 Naming restrictions are implemented by both the Azure platform and AKS. If a resource name or parameter breaks one of these restrictions, an error is returned that asks you provide a different input. The following common naming guidelines apply:
 
 * Cluster names must be 1-63 characters. The only allowed characters are letters, numbers, dashes, and underscores. The first and last character must be a letter or a number.
-* The AKS *MC_* resource group name combines resource group name and resource name. The auto-generated syntax of `MC_resourceGroupName_resourceName_AzureRegion` must be no greater than 80 chars. If needed, reduce the length of your resource group name or AKS cluster name.
+* The AKS Node/*MC_* resource group name combines resource group name and resource name. The auto-generated syntax of `MC_resourceGroupName_resourceName_AzureRegion` must be no greater than 80 chars. If needed, reduce the length of your resource group name or AKS cluster name. You may also [customize your node resource group name](cluster-configuration.md#custom-resource-group-name)
 * The *dnsPrefix* must start and end with alphanumeric values and must be between 1-54 characters. Valid characters include alphanumeric values and hyphens (-). The *dnsPrefix* can't include special characters such as a period (.).
+* AKS Nodepool names must be all lowercase and be 1-11 characters for linux nodepools and 1-6 characters for windows nodepools. The name must start with a letter and the only allowed characters are letters and numbers.
 
 ## I'm receiving errors when trying to create, update, scale, delete or upgrade cluster, that operation is not allowed as another operation is in progress.