Emphasise library(duckplyr) in README

[hadley]

I think you should start with library(duckplyr) and then tell folks that if they don't want to attach it (i.e. if they're writing a package), they can instead use as_duckplyr_tibble().

And most of those details should probably live in the getting started vignette, rather than in the README.

[krlmlr]

DoD:

• Rewrite README to be as short and sweet as dplyr's README
• Review
• Create "Getting started" vignette from what's now gone
• Review