Update README to be user-facing, not contributor-facing

[mfisher87]

Thanks for calling this out @tylere -- when contributors clone the repository into their JupyterHub server, they may immediately look at README.md. It's currently entirely focused on contributors, but it should be more user-facing.

• Extract contributing information to a CONTRIBUTING.md
• What should go in the README itself?

[tylere]

I think the README in the root folder should point potential users on where to start. Something like:

Workshop participants - Open up the materials/01-working-01-working-in-jupyterhub/index.md, right-click to open the context menu, and select Show Markdown Preview.
Instructors - See the CONTRIBUTING.md and the for-instructors/ folder.