Improve README

Author: n0zawana

README.md

@@ -377,6 +377,7 @@ Examples:
   * `autocommit=1`: `SET autocommit=1`
   * [`time_zone=%27Europe%2FParis%27`](https://dev.mysql.com/doc/refman/5.5/en/time-zone-support.html): `SET time_zone='Europe/Paris'`
   * [`tx_isolation=%27REPEATABLE-READ%27`](https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tx_isolation): `SET tx_isolation='REPEATABLE-READ'`
+    * [`tx_isolation`](https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation) is removed in MySQL 8.0. It is required to use [`transaction_isolation`](https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_transaction_isolation) instead of using `tx_isolation` from MySQL 8.0.
 
 
 #### Examples