Add citation for tskit to DESCRIPTION file

[gregorgorjanc]

According to https://github.com/DavisVaughan/extrachecks?tab=readme-ov-file#you-get-asked-if-there-are-any-references-describing-the-methods-in-your-package

You get asked if there are any “references describing the methods in your package.”
This comment normally comes as the following standard block of text:

If there are references describing the methods in your package, please add these in the description field of your DESCRIPTION file in the form authors (year) <doi:…> authors (year) <arXiv:…> authors (year, ISBN:…) or if those are not available: <https:…> with no space after ‘doi:’, ‘arXiv:’, ‘https:’ and angle brackets for auto-linking. (If you want to add a title as well please put it in quotes: “Title”)

So, should we wait till tskit paper is submitted?