Grouped data vignette

[hadley]

We need a vignette that broadly explains how grouping works, and shows how grouping and the various verbs interact.

• use_vignette("grouping.Rmd")
• Copy in existing content from https://dplyr.tidyverse.org/articles/dplyr.html#grouped-operations (which scheduled for removal)
• Ensure each one table verb has a section with a small example that demonstrates how grouping interacts with its operation
  • un-select() grouping vars
  • arrange() not grouped by default
  • show example of filter to remove entire groups, e.g. filter(df, n() == 1)
• Should try and work with one dataset (starwars?) as much as possible.
• How to access data about the "current" group with cur_group() and friends
• Briefly show the underlying data structure accessed with group_data()

This is a lot to do in a day, so if you don't get it all done, please submit a PR anyway, as your work will make it much easier for us to finish off.