Add a note about mysql_native_password next to our use of it

mysql/stack.yml

@@ -5,6 +5,8 @@ services:
 
   db:
     image: mysql
+    # NOTE: use of "mysql_native_password" is not recommended: https://dev.mysql.com/doc/refman/8.0/en/upgrading-from-previous-series.html#upgrade-caching-sha2-password
+    # (this is just an example, not intended to be a production configuration)
     command: --default-authentication-plugin=mysql_native_password
     restart: always
     environment: