Document how to find size of database

https://docs.gitlab.com/omnibus/settings/database.html#upgrade-packaged-postgresql-server

• You will need to have sufficient disk space for two copies of your database. Do not attempt to upgrade unless you have enough free space available. If the partition where the database resides does not have enough space (default location is /var/opt/gitlab/postgresql/data), you can pass the argument --tmp-dir $DIR to the command.

How can I easily determine the size of my database?