Add llama2 readme in examples/README (#2992)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/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