add preview note

Author: duongau

articles/frontdoor/tier-upgrade.md

@@ -13,6 +13,10 @@ ms.author: duau
 
 Azure Front Door supports upgrading from Standard to Premium tier for more advanced capabilities and an increase in quota limits. The upgrade won't cause any downtime to your services or applications. For more information about the differences between Standard and Premium tier, see [Tier comparison](standard-premium/tier-comparison.md).
 
+> [!IMPORTANT]
+> Upgrading Azure Front Door tier 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.
+
 This article will walk you through how to perform the tier upgrade on the configuration page of a Front Door Standard profile. Once upgraded, you'll be charge for the Azure Front Door Premium monthly base fee at an hourly rate.
 
 > [!IMPORTANT]