Revise README for NServiceBus sunset announcement

Update README to reflect the sunset of NServiceBus.AzureFunctions.InProcess.ServiceBus and provide migration instructions.

Author: johnsimons

README.md

@@ -1,6 +1,7 @@
 > [!CAUTION] 
 > **NServiceBus.AzureFunctions.InProcess.ServiceBus has been officially sunsetted.**  
-> Microsoft announced that .NET 8 will be [the last release supporting the in-process hosting model](https://techcommunity.microsoft.com/t5/apps-on-azure-blog/net-on-azure-functions-august-2023-roadmap-update/ba-p/3910098).  
+> Microsoft announced that .NET 8 will be [the last release supporting the in-process hosting model](https://techcommunity.microsoft.com/t5/apps-on-azure-blog/net-on-azure-functions-august-2023-roadmap-update/ba-p/3910098).
+> We will continue to provide support and address critical fixes during the sunset period, but no new features will be added.  
 > New projects should use the [isolated worker model](https://docs.particular.net/nservicebus/hosting/azure-functions-service-bus/) instead. Follow [these instructions](https://docs.particular.net/nservicebus/upgrades/azure-functions-service-bus-in-process-isolated-worker) to migrate.
 
 # NServiceBus.AzureFunctions.InProcess.ServiceBus