I like how clean it is! I'd suggest to set a max width of around 6-700 pixels for the contenteditable div, this will give a better readability. It would be able if you could use titles and lists and other html elements in the text.
Saving the document could be done by sending the content to the server with an ajax request and then create a txt file on the server and send it back to the user
http://www.tizag.com/phpT/filecreate.php
I suggest you use the current time to create the file to avoid file-swap when two people save in a small time lapse.
$ourFileName = "simple-editor-".time().".txt";
$ourFileHandle = fopen($ourFileName, 'w') or die("can't open file");
$myFile = $ourFileName;
$fh = fopen($myFile, 'w') or die("can't open file");
$stringData = "Bobby Bopper\n";
fwrite($fh, $stringData);
fclose($fh);
If you're really into trying some backend codes i'd love if there was authentication and the possibility to save files on the server.
I think you can create a txt file with js as well, but I would go with php