Start community-provided dataset docs 
Oh, really cool @sshleifer!
Continuation of #736 with clean fork.

#### Old description
This is what I did to get the pseudo-labels updated. Not sure if it generalizes, but I figured I would write it down. It was pretty easy because all I had to do was make properly formatted directories and change URLs.

In slack @thomwolf called it a user-namespace dataset, but the docs call it community dataset.
I think the first naming is clearer, but I didn't address that here.

I didn't add metadata, will try that.