Update README with WIP note (#58)

Author: davidmrdavid

README.md

@@ -1,5 +1,9 @@
 # Durable Functions for PowerShell
 
+⚠️ **This SDK is currently under active development, is yet to be announced, and is not ready for production use.** ⚠️
+
+> Note that this project is **not** yet announced as an official part of the [Durable Functions](https://docs.microsoft.com/azure/azure-functions/durable/durable-functions-overview) project for Azure Functions, it is simply being developed in the open. If you are looking for official support for the PowerShell Durable Functions SDK, please see [this repo](https://github.com/Azure/azure-functions-powershell-worker).
+
 This repo hosts the standalone Durable Functions SDK for PowerShell. Compared to the Durable Functions SDK that comes built-in with the Azure Functions PowerShell worker, this standalone SDK contains performance enhancements, more features, and key bug fixes that would have required a breaking release. For more information on this release, please see [this article](TODO).
 
 > The standalone Durable Functions SDK implementation is currently in **preview**. For production workloads, we recommend continuing to use the built-in Durable Functions SDK.