mysql-compatibility.md: Add note about JOIN on subquery (#20309) (#20353)

Author: ti-chi-bot

mysql-compatibility.md

@@ -71,6 +71,7 @@ You can try out TiDB features on [TiDB Playground](https://play.tidbcloud.com/?u
 + Descending Index [#2519](https://github.com/pingcap/tidb/issues/2519)
 + `SKIP LOCKED` syntax [#18207](https://github.com/pingcap/tidb/issues/18207)
 + Lateral derived tables [#40328](https://github.com/pingcap/tidb/issues/40328)
++ JOIN ON subquery [#11414](https://github.com/pingcap/tidb/issues/11414)
 
 ## Differences from MySQL