Added some use case in the readme

[DanChaltiel]

Here is a proposal for use cases in the documentation.

This is in line with https://github.com/nanxstats/ggsci/issues/12

I never use base plots so you might be able to come with a better syntax (I thought it was nice to show it too).

I edited this directly on GitHub so I did not knit the Rmd to md.

I used eval=FALSE but you might choose otherwise.

As it seems that you are not using GitHub actions to generate pkgdown (you should give it a try, it is very nice!), you might also need to re-run your workflow manually.

Thanks again for your package!