[SYSADMIN] Server post-installation TODO

The server post-installation script seems to have run successfully. Remember to
perform the following tasks:

- once when mail system set-up seems stable, in
  config/dotfiles_user_server/getmail/getmailrc, set [options] delete = true

- ensure the following DNS TXT record for @: v=spf1 mx -all

- run (as root) config/setup_opendkim.sh $selector to set up system for DKIM key
  signing, with a second parameter $keyfile if a key already exists; without
  second parameter, this will generate a new key and print the DNS record to add