Update package README using R markdown

[hturner]

Improve the README to give a bit more information about the package, e.g.

• current data available
• additional features (API functions, dashboard, if added)
• example of how to install and basic use (could use same example as Add example (plot?) to tdor help file #21)

Using usethis::use_readme_rmd() is a good idea as it creates a template and also a git hook to make sure the .Rmd file is rendered to .md before being committed.

[hturner]

Just discovered there is a GitHub action to update the README too: https://github.com/r-lib/actions/tree/master/examples#render-readme, so maybe we can forget the git hook?

[hturner]

Helper functions to use GitHub actions: https://usethis.r-lib.org/reference/github_actions.html