Post on vctrs for people who don't use vctrs

[batpigandme]

As vctrs becomes the source of more "stuff" in tidyverse packages, users are bound to run into some errors that reference vctrs (e.g. tidyverse/tidyr#750). I think it might be helpful to have a ~crib sheet for people who definitely won't be working with vctrs, but might see these types of errors.

This could be a vctrs vignette, but I think there might be value in having this outside of the vctrs repo, since it would intentionally gloss over aspects that someone working with vectors would want to know.

I think we've done a pretty good job of this type of thing with tidyselect (though it still causes confusion from time to time). It's essentially one level beyond looking at the source code for the user-facing package that are attached directly.

This might also tie in with reading the traceback. So, that might be a more generic post in addition to or instead of this one.