Update README.md with TorchTune reference

We'd estimate communities like to try ExecuTorch with their own tuned models. Add a reference to TorchTune so that users can use all PyTorch native support in the entire flow.

Author: iseeyuan

examples/models/llama2/README.md

@@ -91,6 +91,8 @@ If you want to deploy and run a smaller model for educational purposes. From `ex
     python -m examples.models.llama2.tokenizer.tokenizer -t tokenizer.model -o tokenizer.bin
     ```
 
+If you want to fine tune your model based on a specific dataset, PyTorch team provides a native-Pytorch library, [TorchTune](https://github.com/pytorch/torchtune), for easily authoring, fine-tuning and experimenting with LLMs.
+
 ## Step 3: Evaluate model accuracy
 
 > Forewarning: Model evaluation without a GPU may take a long time, especially on larger models.