Update README.md to ask user to double check python env (#3782)

Summary:
As titled.

Pull Request resolved: #3782

Reviewed By: tarun292, mcr229

Differential Revision: D57983213

Pulled By: larryliu0820

fbshipit-source-id: 4a845122cd5d7c94d697932e356e6c5f786014cf
(cherry picked from commit ccce5fa)

Author: larryliu0820

examples/models/llama2/README.md

@@ -53,9 +53,12 @@ Performance was measured on the Samsung Galaxy S22, S24, and One Plus 12 devices
 - For Llama7b, your device may require at least 32GB RAM. If this is a constraint for you, please try the smaller stories model.
 
 ## Step 1: Setup
-1. Follow the [tutorial](https://pytorch.org/executorch/main/getting-started-setup) to set up ExecuTorch
+> :warning: **double check your python environment**: make sure `conda activate <VENV>` is run before all the bash and python scripts.
+
+1. Follow the [tutorial](https://pytorch.org/executorch/main/getting-started-setup) to set up ExecuTorch. For installation run `./install_requirements.sh --pybind xnnpack`
 2. Run `examples/models/llama2/install_requirements.sh` to install a few dependencies.
 
+
 ## Step 2: Prepare model
 
 ### Option A: Download and export llama2 7B model