Update README to include instructions for using dashboard

[mnlevy1981]

Now that the notebooks create png files rather than inline plots, the README.md file should walk through all the steps required to generate the images, create the csv file for the dashboard, and then use the dashboard notebook. It should also include instructions for creating new plot classes, and how to incorporate those plots in the dashboard.

Note that, in addition to making sure (hires-marbl) is up-to-date with environment.yaml, a user's (base) environment needs jupyterlab>=3, ipywidgets, and hvplots if that is the environment used to launch jupyter lab