Update how-to-create-manage-compute-instance.md

Describe az login command usage

Author: rastala

articles/machine-learning/how-to-create-manage-compute-instance.md

@@ -480,6 +480,9 @@ def get_access_token_msi(resource):
 arm_access_token = get_access_token_msi("https://management.azure.com")
 ```
 
+> [!NOTE]
+> To use Azure CLI with the managed identity for authentication, specify the identity client ID as the username when logging in: ```az login --identity --username $DEFAULT_IDENTITY_CLIENT_ID```.
+
 ## Add custom applications such as RStudio (preview)
 
 You can set up other applications, such as RStudio, when creating a compute instance. Follow these steps in studio to set up a custom application on your compute instance