1. There are extensions out there like k2 notify.
2. you can try to hardcode in itemform.php (create override first), something like below. (I'm not a coder)
<?php
$to = "youremail";
$subject = "yoursubject";
$txt = $this->row->title;
$headers = "From: youremail";
mail($to,$subject,$txt,$headers);
?>
Issue here you won't know which article has been changed / added. You will receive an email (each time someone open the form to submit an item but who does not necessarily submit) and then you check which k2 items are unpublished in back-end.
3. You can enable use User Actions Log Options in joomla user management for admin account. This can be a bit spammy since you might received all kinds of notifications and not necessarily when user submits.