Inform default locations of STORDIR and ARCHDIR in docs and how to change the config.

[didacog]

The default directory where DRLM store the backups is /var/lib/drlm/arch (ARCHDIR=$VAR_DIR/arch in default.conf).

You can mount your filesystem on this directory without any configuration changes, or if you want to change it, just edit /etc/drlm/local.conf and set:

ARCHDIR=/new/location

Can do the same for STORDIR but as this directory does not use lot of filesystem space is recommended to keep default config.