"Community content is available under CC-BY-SA unless otherwise noted."
For the attribution part, it was made by anonymous poster(s) on the /x/ board of 4chan. That means you don't have anyone to actually attribute the work to, but you still need to link to the CC-BY-SA license and note if you made any changes to the story itself.
Really though, if you're going to make a channel out of story readings, you should do this research on your own. It'll save you trouble down the line.