Hi,
The vignette on column types has been incorporated into a new readr vignette. In your R session you can know use vignette("readr")` to read it. There is a chapter on Column specification and on Overriding the defaults that can help you. Know that, you can also read this vignette online on CRAN website
https://cran.r-project.org/web/packages/readr/vignettes/readr.html
Link to vignette has been change in the code of package but website has not been updated yet with the fix.
@jennybryan, to help, I opened an issue on this in readr github repository to have it listed.
Thanks for reporting @jabowery