add `How to cite` and enable `citation()`

[Yingjie4Science]

CITATION files for R packages

This is typically located under the folder at inst/CITATION

• [x] To be edited upon publication

Or, You can add a CITATION file to your repository

to help users correctly cite your software. See https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

[Yingjie4Science]

done