Mention jupyter/docker stacks as an alternative to rocker/binder

rocker/binder contains some Jupyter things, but the rocker project
as a whole is not focused on Jupyter. This links them out to the
jupyter/docker-stacks project, which is more Jupyter focused to
serve users who are looking for more Jupyter focused container images.
I think a lot of communities / users use a combination of Python (from
various sources) and R (from the rocker project) already anyway.

I link to the rocker project from jupyter/docker-stacks since
last month (jupyter/docker-stacks#2065),
and one of the maintainers asked if I could provide a link
back as well.

images/versioned/binder.md

@@ -74,3 +74,9 @@ execute `/init` command with the root user specified.
 ```sh
 docker run --rm -ti -p 8787:8787 --user root rocker/binder /init
 ```
+
+## Alternatives
+
+If you are *primarily* interested in using the Jupyter ecosystem of tools, consider using
+the images from the [jupyter/docker-stacks](https://jupyter-docker-stacks.readthedocs.io/)
+project instead.