docs: add MySQLChatMessageHistory to readme (#33)

* docs: add MySQLChatMessageHistory to readme * docs: fix error --------- Co-authored-by: Averi Kitsch <akitsch@google.com>

Author: loeng2023

README.md

@@ -53,7 +53,7 @@ See the full [Document Loader][loader] tutorial.
 
 ## Chat Message History Usage
 
-Use `ChatMessageHistory` to store messages and provide conversation history to LLMs.
+Use [ChatMessageHistory](https://python.langchain.com/docs/modules/memory/chat_messages/) to store messages and provide conversation history to LLMs.
 
 ```python
 from langchain_google_cloud_sql_mysql import MySQLChatMessageHistory, MySQLEngine