[related_resources] add haystack to prompting libraries & tools (#682)

Co-authored-by: simonpfish <simonpfish@gmail.com>

related_resources.md

@@ -21,6 +21,7 @@ People are writing great tools and papers for improving outputs from GPT. Here a
 - [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.
 - [LMQL](https://lmql.ai): A programming language for LLM interaction with support for typed prompting, control flow, constraints, and tools.
+- [Haystack](https://github.com/deepset-ai/haystack): Open-source LLM orchestration framework to build customizable, production-ready LLM applications in Python.
 
 ## Prompting guides