add arthur shield to external libraries section

README.md

@@ -84,7 +84,8 @@ People are writing great tools and papers for improving outputs from GPT. Here a
 - [PromptPerfect](https://promptperfect.jina.ai/prompts): A paid product for testing and improving prompts.
 - [Weights & Biases](https://wandb.ai/site/solutions/llmops): A paid product for tracking model training and prompt engineering experiments.
 - [OpenAI Evals](https://github.com/openai/evals): An open-source library for evaluating task performance of language models and prompts.
-- [LlamaIndex](https://github.com/jerryjliu/llama_index): A Python library for augmenting LLM apps with private data (personal, organizational).
+- [LlamaIndex](https://github.com/jerryjliu/llama_index): A Python library for augmenting LLM apps with data.
+- [Arthur Shield](https://www.arthur.ai/get-started): A paid product for detecting toxicity, hallucination, prompt injection, etc.
 
 ### Prompting guides