Add references to related LLM framework projects (LangChain and LlamaIndex)

Summary

This issue proposes adding references to related LLM framework projects in the Haystack repository.

Related Projects

1. LangChain - https://github.com/langchain-ai/langchain

   • A framework for developing applications powered by language models
   • Currently has ~233 open issues

2. LlamaIndex - https://github.com/run-llama/llama_index

   • A data framework for LLM applications to ingest, structure, and access private or domain-specific data
   • Currently has ~230 open issues

Comparison

Open issue counts as of March 2026

Suggestion

Consider adding a section in the README.md or creating a RELATED_PROJECTS.md file to document these related projects. This would help users understand the LLM framework ecosystem and how Haystack fits into it.

Benefits

• Helps developers understand the LLM framework landscape
• Provides context for users comparing different frameworks
• Demonstrates Haystack's position in the ecosystem (notably, Haystack has fewer open issues, which may indicate good maintenance)

What do you think?

Start by reviewing README.md to see where related projects are documented, then compare that with the proposed RELATED_PROJECTS.md option. The work is complete when the Haystack repository includes clear references to LangChain and LlamaIndex with their supplied descriptions and links.