Documentation: add the hint that you can use global options

[DanChaltiel]

Hi Nan,

Thanks for this great package!

I just discovered that you can use global options to set the scale for all your plots:

options(ggplot2.discrete.colour=ggsci::scale_colour_lancet)
options(ggplot2.discrete.fill=ggsci::scale_fill_lancet)

I think this might be worth showing in the doc and in the pkgdown website.

[nanxstats]

Good - a PR is welcomed!

[nanxstats]

Fixed by https://github.com/nanxstats/ggsci/commit/78a24cea89c994cc1ee6fd1f7fcb3109e1c376f7