release notes: Updated README.md installation for MacOS (pytorch#7457)

Update xnnpack_README.md

To install XNNPACK on macOS, you need to run the ./install_requirements.sh script with the --pybind xnnpack option. This ensures that the necessary dependencies are correctly configured for your environment.
@pytorchbot label "release notes: Updated README.md installation for MacOS"

examples/demo-apps/apple_ios/LLaMA/docs/delegates/xnnpack_README.md

@@ -34,7 +34,10 @@ Install dependencies
 ```
 ./install_requirements.sh
 ```
-
+Optional: Use the --pybind flag to install with pybindings.
+```
+./install_requirements.sh --pybind xnnpack
+```
 ## Prepare Models
 In this demo app, we support text-only inference with up-to-date Llama models and image reasoning inference with LLaVA 1.5.
 * You can request and download model weights for Llama through Meta official [website](https://llama.meta.com/).