Add info about postgres upgrade

Merged Achilleas Pipinellis requested to merge archlinux_pg_upgrade into master

If system postgres is updated to a new minor version, we get an error message: FATAL: database files are incompatible with server. This MR adds some simple steps to upgrade the postgres data to the new version. Some steps might be specific to Arch Linux, but that will give a rough idea how to get things going.