Add notice to new WinML (#534)

Author: andrewleader

README.md

@@ -1,5 +1,7 @@
 # Windows Machine Learning
 
+> ⚠️ **IMPORTANT**<br/>For the latest documentation about Windows Machine Learning, see [What is Windows ML](/windows/ai/new-windows-ml/overview). That documentation describes APIs that are in the **Microsoft.Windows.AI.MachineLearning** namespace, which ships in the Windows App SDK. Those APIs supersede the ones documented here, which are in the **Windows.AI.MachineLearning** namespace, and were shipped in 2018.
+
 Windows Machine Learning is a high-performance machine learning inference API that is powered by [ONNX Runtime](https://onnxruntime.ai/) and [DirectML](https://docs.microsoft.com/en-us/windows/ai/directml/dml).