mention vroom as the parsing engine in readr (in the readme)

[luisDVA]

Hi,
A quick suggestion that came up while teaching:

The readme mentions that "vroom has nearly all of the parsing features of readr for delimited and fixed width files" and while this is true; this part of the readme feels out of date with readr (2nd ed) now calling vroom::vroom() by default.

It may be worth mentioning this link between the two, and I'm happy to send a PR for this if needed.
best,
Luis

[jennybc]

Thanks! Yeah, various corners of vroom and especially its docs need some love. I'll take a crack at this, as I'm actually working on this right now.