fix R CMD CHECK note

[pawelru]

* checking package dependencies ... NOTE
Vignette dependency required without any vignettes: ‘knitr’

[pawelru]

I have read the CLA Document and I hereby sign the CLA

[donyunardi]

Might as well fix this note by adding SECURITY.md to .Rbuildignore?

❯ checking top-level files ... NOTE
  Non-standard file/directory found at top level:
    ‘SECURITY.md’

[github-actions]

✅ All contributors have signed the CLA
_{Posted by the CLA Assistant Lite bot.}

[pawelru]

Thanks. Fixed this as well as the one about title case (turned on only for --as-cran)

[pawelru]

I have read the CLA Document and I hereby sign the CLA