Add message that subscription IDs need to be non-empty GUIDs

Author: juliamwang6

articles/azure-resource-manager/management/azure-subscription-service-limits.md

@@ -59,6 +59,8 @@ The following limits apply when you use Azure Resource Manager and Azure resourc
 
 [!INCLUDE [azure-subscription-limits-azure-resource-manager](~/reusable-content/ce-skilling/azure/includes/azure-subscription-limits-azure-resource-manager.md)]
 
+Note that subscription IDs must be non-empty GUIDs.
+
 ### Azure resource group limits
 
 [!INCLUDE [azure-resource-groups-limits](../../../includes/azure-resource-groups-limits.md)]