Код:

function addLink() {

    var selection = window.getSelection(),
        pagelink = '<br /><br /> Read more at: ' + document.location.href,
        copytext = selection + pagelink,
        newdiv = document.createElement('div');

    newdiv.style.position = 'absolute';
    newdiv.style.left = '-99999px';

    document.body.appendChild(newdiv);
    newdiv.innerHTML = copytext;
    selection.selectAllChildren(newdiv);

    window.setTimeout(function () {
        document.body.removeChild(newdiv);
    }, 100);
}

document.addEventListener('copy', addLink);