Update readme

description

Update the readme (closes #47 (closed)):

• Reference the next paper.
• Include multi_tox() in examples.
• Add example plots and console output.

Other:

• Add "fnd" to roles of the UFZ.

system details

• R Version: 4.0.2
• RStudio Version: 1.3.1054
• OS Version: Windows 10 x64

checklist

• This merge introduces new features.
  • new features are documented
  • new features have tests
• update documentation including examples (if necessary)
• update and polish NEWS.md
• update version in DESCRIPTION
• update date in DESCRIPTION
• no remaining TODO, FIXME, or debug prints anywhere in the source files
• devtools::document() and commit the updated .Rd files
• devtools::check() without errors or warnings