2022 Update

More complex solution that handles rich text formatting. The 2020 solution is still relevant if you only deal with plain text.

const copyListener = (e) => {
  const range = window.getSelection().getRangeAt(0),
    rangeContents = range.cloneContents(),
    pageLink = `Read more at: ${document.location.href}`,
    helper = document.createElement("div");

  helper.appendChild(rangeContents);

  event.clipboardData.setData("text/plain", `${helper.innerText}\n${pageLink}`);
  event.clipboardData.setData("text/html", `${helper.innerHTML}<br>${pageLink}`);
  event.preventDefault();
};
document.addEventListener("copy", copyListener);

#richText {
  width: 415px;
  height: 70px;
  border: 1px solid #777;
  overflow: scroll;
}

#richText:empty:before {
  content: "Paste your copied text here";
  color: #888;
}

<h4>Rich text:</h4>
<p>Lorem <u>ipsum</u> dolor sit <b>amet</b>, consectetur <i>adipiscing</i> elit.</p>
<h4>Plain text editor:</h4>
<textarea name="textarea" rows="5" cols="50" placeholder="Paste your copied text here"></textarea>
<h4>Rich text editor:</h4>
<div id="richText" contenteditable="true"></div>

2020 Update

Solution that works on all recent browsers.

Note that this solution will strip rich text formatting (such as bold and italic), even when pasting into a rich text editor.

document.addEventListener('copy', (event) => {
  const pagelink = `\n\nRead more at: ${document.location.href}`;
  event.clipboardData.setData('text/plain', document.getSelection() + pagelink);
  event.preventDefault();
});

Lorem ipsum dolor sit <b>amet</b>, consectetur <i>adipiscing</i> elit.<br/>
<textarea name="textarea" rows="7" cols="50" placeholder="paste your copied text here"></textarea>

[Older post – before the 2020 update]

There are two main ways to add extra info to copied web text.

1. Manipulating the selection

The idea is to watch for the copy event, then append a hidden container with our extra info to the dom, and extend the selection to it.
This method is adapted from this article by c.bavota. Check also jitbit‘s version for more complex case.

• Browser compatibility: All major browsers, IE > 8.
• Demo: jsFiddle demo.
• Javascript code:

    function addLink() {
        //Get the selected text and append the extra info
        var selection = window.getSelection(),
            pagelink = '<br /><br /> Read more at: ' + document.location.href,
            copytext = selection + pagelink,
            newdiv = document.createElement('div');

        //hide the newly created container
        newdiv.style.position = 'absolute';
        newdiv.style.left="-99999px";

        //insert the container, fill it with the extended text, and define the new selection
        document.body.appendChild(newdiv);
        newdiv.innerHTML = copytext;
        selection.selectAllChildren(newdiv);

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

    document.addEventListener('copy', addLink);

2. Manipulating the clipboard

The idea is to watch the copy event and directly modify the clipboard data. This is possible using the clipboardData property. Note that this property is available in all major browsers in read-only; the setData method is only available on IE.

• Browser compatibility: IE > 4.
• Demo: jsFiddle demo.
• Javascript code:

    function addLink(event) {
        event.preventDefault();

        var pagelink = '\n\n Read more at: ' + document.location.href,
            copytext =  window.getSelection() + pagelink;

        if (window.clipboardData) {
            window.clipboardData.setData('Text', copytext);
        }
    }

    document.addEventListener('copy', addLink);