update COC

[sinarueeger]

ropensci has a new guideline to add a coc:
https://devguide.ropensci.org/collaboration.html#code-of-conduct

• removed COC file
• replaced COC snippet in README.Rmd and re-knitted the file.

[codecov-commenter]

Codecov Report

Merging #110 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #110   +/-   ##
=======================================
  Coverage   64.46%   64.46%           
=======================================
  Files          13       13           
  Lines         318      318           
=======================================
  Hits          205      205           
  Misses        113      113           

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[jooolia]

thanks for updating this to be in line with ropenSci's guidelines.