add preview note

Author: duongau

articles/frontdoor/managed-identity.md

@@ -13,6 +13,10 @@ ms.author: duau
 
 Azure Front Door also supports using managed identities to access Key Vault certificate. A managed identity generated by Azure Active Directory (Azure AD) allows your Azure Front Door instance to easily and securely access other Azure AD-protected resources, such as Azure Key Vault. Azure manages this identity, so you don't have to create or rotate any secrets. For more information about managed identities, see [What are managed identities for Azure resources?](../active-directory/managed-identities-azure-resources/overview.md).
 
+> [!IMPORTANT]
+> Migration identity for Azure Front Door is currently in PREVIEW.
+> See the [Supplemental Terms of Use for Microsoft Azure Previews](https://azure.microsoft.com/support/legal/preview-supplemental-terms/) for legal terms that apply to Azure features that are in beta, preview, or otherwise not yet released into general availability.
+
 > [!NOTE]
 > Once you enable managed identities in Azure Front Door and grant proper permissions to access Key Vault, Azure Front Door will always use managed identities to access Key Vault for customer certificate. 
 >