docs: update README to reflect file structure changes

Author: srtaalej

README.md

@@ -85,11 +85,14 @@ Every incoming request is routed to a "listener". This directory groups each lis
 
 Configures the new Slack Assistant features, providing a dedicated side panel UI for users to interact with the AI chatbot. This module includes:
 
-`assistant.py`, which contains two listeners:
+`assistant_thread_started.py`, which contains:
 *  The `@assistant.thread_started` listener receives an event when users start new app thread.
+
+`message.py`, which contains:
 *  The `@assistant.user_message` listener processes user messages in app threads or from the app **Chat** and **History** tab.
 
-`ai/llm_caller.py`, which handles OpenAI API integration and message formatting. It includes the `call_llm()` function that sends conversation threads to OpenAI's models.
+### `ai/`
+`llm_caller.py`, which handles OpenAI API integration and message formatting. It includes the `call_llm()` function that sends conversation threads to OpenAI's models.
 
 ## App Distribution / OAuth