guess_readme() matches a single README file

[bastistician]

Fixes #189 and fixes #190.

guess_readme() now matches README.Rmd and README.md as well as their lower-/mixed-case variants (including readme.rmd which hasn't been matched before), but no other files such as README.Rmd~ or DO_NOT_README.md. The function either returns a single readme_path or NULL. I have added an appropriate test for this behaviour.

In the rare event where there exist multiple mixed-case README.md files on Unix-alikes, the function returns the first match (which is locale-dependent). I have decided not to warn about that since codemetar is generally non-verbose about its guesses.

I've also added myself to the DESCRIPTION using desc::desc_add_me() as suggested by @maelle (#190 (comment)).

[maelle]

Thanks a lot @bastistician! I see the failure on Travis is due to rOpenSci review badges and underlines the need for #175.

ignore.case = TRUE is so elegant! And I'm glad desc::desc_add_me() worked 😸