chore: add notice about transition to stable (#967)

Author: mwwoda

README.md

@@ -2,9 +2,15 @@
   <img src="https://github.com/box/sdks/blob/master/images/box-dev-logo.png" alt= “box-dev-logo” width="30%" height="50%">
 </p>
 
+# Deprecation notice
+
+This version of the Box .NET SDK is under maintenance mode, and will be deprecated soon, only critical security updates and bug fixes will be provided. We recommend using the new version Box .NET SDK, which can be found at [box/box-dotnet-sdk-gen](https://github.com/box/box-dotnet-sdk-gen)
+
+You can find the migration guide [here](https://github.com/box/box-dotnet-sdk-gen/blob/main/migration-guide.md) for transitioning from Box .NET SDK v5.x to the new `box-sdk-gen` package. If you have any questions, please create an issue in the new repository or reach out to [Box Developer Support](https://developer.box.com/support/).
+
 # Box Windows V2 SDK
 
-[![Project Status](http://opensource.box.com/badges/active.svg)](http://opensource.box.com/badges)
+[![Project Status](http://opensource.box.com/badges/stable.svg)](http://opensource.box.com/badges)
 ![Platform Framework](https://img.shields.io/badge/.NET%20Framework-%3E%3D4.6.2-blue)
 ![Platform Core](https://img.shields.io/badge/.NET%20Core-%3E%3D2.0-blue)
 [![License](https://img.shields.io/badge/license-Apache2-blue)](https://raw.githubusercontent.com/box/box-windows-sdk-v2/main/LICENSE)