Add promptfoo to tools list

Author: mldangelo

pages/tools.en.mdx

@@ -57,6 +57,7 @@
 - [Promptmetheus](https://promptmetheus.com)
 - [PromptPerfect](https://promptperfect.jina.ai/)
 - [Promptly](https://trypromptly.com/)
+- [promptfoo](https://github.com/promptfoo/promptfoo) - Test and evaluate LLM applications. Compare prompts, models, and RAG pipelines. Red team with adversarial attacks and integrate into CI/CD.
 - [PromptSource](https://github.com/bigscience-workshop/promptsource)
 - [PromptTools](https://github.com/hegelai/prompttools)
 - [Scale SpellBook](https://scale.com/spellbook)