add description for RETURNING

Source Code: [Main.java](./Main.java)

There is a non-standard SQL `RETURNING`:

Offical Document: Search `RETURNING` in https://www.postgresql.org/docs/13/sql-insert.html

It's very useful when you are using serial as primary key as most relational database design dose. If you want to retrive the auto-increment id of the row you inserted just now, simply append `RETURNING column_name` to the INSERT clause.