Some times you want to see the original version of the article you're reading. Kindle devices are not very convinient when it comes to browsing the web. Of couse you can always copy the URL calibre displays in the navbar, but if you have a smartphone, it's much easier to scan a QR code (2d barcode) instead. The following snippet is a skeleton of recipe which adds a QR code to the bottom of the article with its URL.
https://gist.github.com/guyru/d35712edcc5b6adf364c