Update README to specify that users should also install azure-identity (Azure#29343)

Author: kdestin

sdk/ml/azure-ai-ml/README.md

@@ -28,6 +28,7 @@ Install the Azure ML client library for Python with [pip][pip_link]:
 
 ```bash
 pip install azure-ai-ml
+pip install azure-identity
 ```
 
 ### Authenticate the client

sdk/ml/azure-ai-ml/samples/README.md

@@ -29,6 +29,7 @@ These samples provide example code for additional scenarios commonly encountered
 
    ```bash
     pip install azure-ai-ml
+    pip install azure-identity
     ```
 
 2. Clone or download this sample repository