Update Jupyterhub docs to include documentation on IRKernel

[monaghaa]

I updated the JH docs in branch "jupyter_202005". Specific change was adding a description of how to create a kernel for an "R" environment (which is different than creating a kernel for a python environment).

https://github.com/ResearchComputing/Documentation/blob/jupyter_202005/docs/gateways/jupyterhub.md

requesting that this be pulled into Master branch when possible.