DOC: s3fs is required when using `read_csv` with an S3 URI
#### Location of the documentation

[pandas.read_csv](https://dev.pandas.io/docs/reference/api/pandas.read_csv.html)

#### Documentation problem

I've just noticed that `s3fs` is required when you read an URL from s3. While it is documented that you can read from S3, the implication that you need to install an extra is not documented.

Also, it would be nice if this was a pandas extra in setup.py (e.g. `s3`).