docs: documentation for langchain.py (#849)

pandasai/llm/langchain.py

@@ -1,5 +1,13 @@
 from pandasai.prompts.base import AbstractPrompt
 from .base import LLM
+"""Langchain LLM 
+
+This module is to run LLM using LangChain framework.
+
+Example:
+    Use below example to call LLM
+    >>> from pandasai.llm.langchain import LangchainLLm
+"""
 
 
 class LangchainLLM(LLM):