WIP: Integrate codemeta.json

[katrinleinweber]

See diff ;-)

I have...

• [x] ... run testthat::auto_test_package() and saw now errors.

• [x] ... updated the changelog / NEWS.md in the Unreleased section, if my PR introduces user-facing changes.

• [ ] ... incremented the minor or patch version number in DESCRIPTION, if my PR is a new feature or a bug-fix.

• ... added/updated tests to /tests/testthat/ for new features or bug-fixes.

• ... read (at least) the LICENCSE (short version) and agreed that it will apply to my contribution as well.

[katrinleinweber]

Since https://github.com/ropensci/codemetar/issues/57 & #30 however, I'm sceptical of adding another file which mostly duplicates information and can be easily generated by anyone who cares.

Currently, I consider How to cite in the ReadMe as more useful.