Copy the following code and paste it into your single.php file, where you want the QR code to be displayed.
If you also want to provide a QR code on pages, paste the code into page.php as well.
<img src="http://api.qrserver.com/v1/create-qr-code/?size=100x100&data=<?php the_permalink(); ?>" alt="QR: <?php the_title(); ?>"/>
Thanks to Kevin Chard for the tip!