Update ref.txt

ref.txt

@@ -2,3 +2,4 @@
 2. https://github.com/NirDiamant/RAG_Techniques/blob/main/all_rag_techniques/simple_csv_rag_with_llamaindex.ipynb
 3. https://medium.com/@sridevi17j/resolving-modulenotfounderror-no-module-named-llama-index-readers-base-in-llama-index-d9f81cb2a9cf
 4. https://github.com/run-llama/llama_index/blob/main/llama-index-core/llama_index/core/readers/file/base.py
+5. https://docs.llamaindex.ai/en/stable/api_reference/readers/