Include richer brushing example

[hadley]

From @cpsievert

Could cover a lot with just scatterplot<->density/barchart. Transient vs persistent selection, absolute vs marginal frequencies, limitations of brushedPoints()/nearPoints() (e.g., when the source of selection is a summary, you likely need to filter raw data via brush limits)

https://twitter.com/cpsievert/status/1259157628686655488