Add read-only restriction

Author: craigshoemaker

articles/azure-functions/functions-deployment-slots.md

@@ -206,6 +206,7 @@ Azure Functions deployment slots have the following limitations:
 
 - The number of slots available to an app depends on the plan. The Consumption plan is only allowed one deployment slot. Additional slots are available for apps running under the App Service plan.
 - Swapping a slot resets keys for apps that have an `AzureWebJobsSecretStorageType` app setting equal to `files`.
+- When slots are enabled, your Functions app is set to read-only mode in the portal.
 
 ## Support levels