Alright, looking at it it doesn't do that like the calendar does, my mistake. But you can enable automatic backups that achieves the same purpose.

Also I'm seeing that you can show contacts from phone storage, but I'm not seeing anywhere to select the file. Only showing contacts from phone storage and backing up to a file is equivalent to read/write from a file. If you figure out how to set the phone storage location let me know, right now I'm using system contacts and automatic backups to a dir synced with syncthing.