Update README cf. usethis

[peterdesmet]

The README.Rmd largely makes use of usethis::use_readme_rmd() conventions, but deviates from it to. It would be good to have this standardized, cf. inbo/etn#483.

Changes include:

• Use lowercase frictionless even at start of sentence (including Zenodo)
• Refer to github.com
• Use pak to install
• Use ``` r for installation code
• Add favicon to website (optional)