add reminder for openai api usage in example

examples/compatible_with_langchain/READMD.md

@@ -6,6 +6,9 @@ to create agents with third-party libraries.
 In this example, we will show how to create an assistant agent with
 LangChain in AgentScope, and interact with user in a conversation.
 
+**Note** we use OpenAI API for LangChain in this example. Developers can 
+modify it according to their own needs. 
+
 ## Install LangChain
 
 Before running the example, please install LangChain by the following command:
@@ -22,5 +25,3 @@ In its `reply` function, developers only need parse the input message and
 wrap the output message into `agentscope.message.Msg` class.
 After that, developers can build the conversation in AgentScope, and the
 `LangChainAgent` is the same as other agents in AgentScope.
-
-