Add custom color scale function for self-defined color ordering

[nanxstats]

Fixes #22.

This PR updates the FAQ vignette by adding a section. It demonstrates a function that returns a custom color scale function with self-defined color selection and ordering.

[wook2014]

I have used remotes::install_github("nanxstats/ggsci") to update my ggsci and Skipping install of 'ggsci' from a github remote, the SHA1 (2f06ac4e) has not changed since last install. but it seemed that scale_color_custom is still unavailable. What the problem is and is there any solutions?

[nanxstats]

@wook2014 scale_color_custom() is not a function that exists in the ggsci namespace. If you are referring to the custom scale function mentioned in the FAQ vignette, it needs to be defined in your working environment first.