add documentation for package name

[corybrunson]

Description of the issue

If i query ?ggalluvial or help(ggalluvial), whether following ggalluvial:: or after attaching ggalluvial to an R session, nothing comes up. The package should have a titular documentation page, as discussed here.

Reproducible example

?ggalluvial::`ggalluvial-package`
#> No documentation for 'ggalluvial-package' in specified packages and libraries:
#> you could try '??ggalluvial-package'
library(ggalluvial)
#> Loading required package: ggplot2
?ggalluvial
#> No documentation for 'ggalluvial' in specified packages and libraries:
#> you could try '??ggalluvial'
?`ggalluvial-package`
#> No documentation for 'ggalluvial-package' in specified packages and libraries:
#> you could try '??ggalluvial-package'

^{Created on 2019-08-20 by the reprex package (v0.2.1)}