v2.0.0 release tweaks (rstudio#531)

* add donttest around longer leaflet example

* colors example should be donttest

* reduce even more examples with \donttest

* update comments with latest builds

Merge pull request rstudio#459 from rstudio/joe/bugfix/empty-sf

Fix rstudio#452: Leaflet returns an error for empty "sf" "dataframe" objects

Include additional licenses; rename LICENSE to LICENSE.note

Add imports to satisfy R CMD check

Don't run previewColors when testing