Add llama2 readme in examples/README (#2992)

Summary:
Pull Request resolved: #2992

We should promote the llama2 page more in https://github.com/pytorch/executorch/tree/main/examples/

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Reviewed By: iseeyuan

Differential Revision: D56018978

fbshipit-source-id: cbbc7bd2ea4ce55e564bd6b4a2900f623599dde6

Author: mergennachin

examples/README.md

@@ -30,14 +30,17 @@ examples
 
 A user's journey may commence by exploring the demos located in the [`portable/`](./portable) directory. Here, you will gain insights into the fundamental end-to-end workflow to generate a binary file from a ML model in [portable mode](../docs/source/concepts.md##portable-mode-lean-mode) and run it on the ExecuTorch runtime.
 
+## Demo of Llama2
+
+[This page](./models/llama2/README.md) demonstrates how to run a Llama 2 7B model on mobile via ExecuTorch. We use XNNPACK to accelerate the performance and 4-bit groupwise PTQ quantization to fit the model on Android and iOS mobile phones.
 
 ## Demo of Selective Build
 
 To understand how to deploy the ExecuTorch runtime with optimization for binary size, explore the demos available in the [`selective_build/`](./selective_build) directory. These demos are specifically designed to illustrate the [Selective Build](../docs/source/kernel-library-selective_build.md), offering insights into reducing the binary size while maintaining efficiency.
 
 ## Demo of ExecuTorch SDK
 
-You will find demos of [ExecuTorch SDK](./sdk/) in the [`sdk/`](./sdk/) directory. The examples focuses on exporting and executing BundledProgram for ExecuTorch model verification, and ETDump generation.
+You will find demos of [ExecuTorch SDK](./sdk/) in the [`sdk/`](./sdk/) directory. The examples focuses on exporting and executing BundledProgram for ExecuTorch model verification and ETDump for collecting profiling and debug data.
 
 ## Demo Apps
 
@@ -63,11 +66,6 @@ You will find demos of [ExecuTorch QNN Backend](./qualcomm) in the [`qualcomm/`]
 
 The [`xtensa/`](./xtensa) directory hosts a demo that showcases the process of exporting and executing a model on Xtensa Hifi4 DSP. You can utilize [this tutorial](../docs/source/build-run-xtensa.md) to guide you in configuring the demo and running it.
 
-
-## Demo of ExecuTorch SDK
-
-You will find demos of [ExecuTorch SDK](./sdk/) in the [`sdk/`](./sdk/) directory. The examples focuses on exporting and executing BundledProgram for ExecuTorch model verification and ETDump for collecting profiling and debug data.
-
 ## Dependencies
 
 Various models and workflows listed in this directory have dependencies on some other packages. You need to follow the setup guide in [Setting up ExecuTorch from GitHub](https://pytorch.org/executorch/stable/getting-started-setup) to have appropriate packages installed.