Prepare package for rOpenSci onboarding

[peterdesmet]

Guide: https://ropensci.github.io/dev_guide/index.html. Things I noticed that might not be covered yet:

• [x] Add codemeta.json using codemetar::write_codemeta(): 5683c0d
• [ ] Use message() and warning() to communicate with the user
• [x] Code style, run styler::style_pkg(filetype = c("R", "Rmd")): dc9dff6
• [x] README: #155
• [x] Add links for authors: e1f117a
• [ ] All packages should have a test suite that covers major functionality of the package. The tests should also cover the behavior of the package in case of errors. See also issue #97
• [ ] For http requests we strongly recommend using httr over RCurl.
• [x] Try goodpractice::gp(): getting