Installing the lib is super damn easy. There are two ways of doing it: downloading the zip and including in your page or fetching it using Bower, the package manager for the frontend and then including from bower components.
As i really belive in the power of organizing the codebase with package managers, i recomend using bower and, then, doing the following after having Bower installed:
$ bower install -s qcode-decoder
Then, inside your .html file, add the following line before your scripts:
Decoding from camera requires, firstly, access to the user's camera. For getting this, QCodeDecoder will ask the user about it (this process is required so that browsers are able to keep it secure) and, if conceived, start searching for a valid QRCode to decode.