Add reference on how to set up the global git config parameter to have main as a default branch

[maelle]

[maelle]

Maybe simply https://github.blog/2020-07-27-highlights-from-git-2-28/

Then, to be sure the option will apply when using gert/usethis, check that gert::libgit2_config()$version >= "1.1". (If not, maybe you can remove libgit2 and then re-install gert (at least on Ubuntu))

Really just a footnote for https://github.com/cynkra/fledge/blob/25ed1faa084f950fe28b2345ab72780cf8613075/vignettes/out/demo.md#L86

[krlmlr]

This feels a bit out of scope here. We could point to the relevant functionality in usethis though.

[krlmlr]

I don't mind a footnote in the demo.