Update README.md

Add note about using context->data to pass user data to connect and disconnect callbacks

README.md

@@ -286,6 +286,7 @@ return `REDIS_ERR`. The function to set the disconnect callback has the followin
 ```c
 int redisAsyncSetDisconnectCallback(redisAsyncContext *ac, redisDisconnectCallback *fn);
 ```
+`ac->data` may be used to pass user data to this callback, the same can be done for redisConnectCallback.
 ### Sending commands and their callbacks
 
 In an asynchronous context, commands are automatically pipelined due to the nature of an event loop.