Add citation to README.md (#48)

Summary:
As titled, adding citation pointing to the Llama 3 paper.

Pull Request resolved: #48

Test Plan:
Updated based on the "BibTeX formatted citation" available on [arXiv](https://arxiv.org/abs/2407.21783):

{F1793588008}

 {F1793588007}

Instead of copying all author names, I'm using "Llama Team, AI @ Meta", hope that's not an issue.

Reviewed By: JFChi

Differential Revision: D60852336

Pulled By: ujjwalkarn

fbshipit-source-id: 3dfb112380e35c768ad0a76e3a9f53ce3df2a505

Llama-Guard3/README.md

@@ -54,6 +54,19 @@ and ethical AI advancements.
 The same license as Llama 3.1 applies: see the [LICENSE](../LICENSE) file, as well
 as our accompanying [Acceptable Use Policy](USE_POLICY.md).
 
+# Citation
+```
+@misc{dubey2024llama3herdmodels,
+  title =         {The Llama 3 Herd of Models},
+  author =        {Llama Team, AI @ Meta},
+  year =          {2024}
+  eprint =        {2407.21783},
+  archivePrefix = {arXiv},
+  primaryClass =  {cs.AI},
+  url =           {https://arxiv.org/abs/2407.21783}
+}
+```
+
 # References
 
 [Research Paper](https://ai.facebook.com/research/publications/llama-guard-llm-based-input-output-safeguard-for-human-ai-conversations/)