CRAN rejection based on "references describing the methods in your package"

I received this cryptic CRAN message :

Thanks,

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.

Please fix and resubmit, and document what was changed in the submission comments.

I don't get it, the DESCRIPTION file seems quite standard, here it is :

Package: inops
Title: Infix Operators for Detection, Subsetting and Replacement
Version: 0.0.1
Authors@R: c(
    person("Antoine", "Fabri", email = "***@gmail.com", role = c("aut", "cre")),
    person("Karolis", "Koncevičius", email = "***@gmail.com", role = "aut")
    )
Description: Infix operators to detect, subset, and replace the elements matched by a given condition.
  The functions have several variants of operator types, including subsets, ranges, regular expressions and others.
  Implemented operators work on vectors, matrices, and lists.
Depends: R (>= 3.1.0)
License: GPL-3
Encoding: UTF-8
LazyData: true
RoxygenNote: 6.1.1
Suggests:
    testthat (>= 2.1.0),
    purrr,
    knitr,
    rmarkdown,
    dplyr,
    nycflights13
URL: https://github.com/moodymudskipper/inops
BugReports: https://github.com/moodymudskipper/inops/issues
VignetteBuilder: knitr
Roxygen: list(markdown = TRUE)

Seems related to bullet points 5 to 7 but I don't see how it's relevant here : https://cran.r-project.org/web/packages/submission_checklist.html

Any idea what's expected from me here ?

The repo if it helps : https://github.com/moodymudskipper/inops