Create CODE_OF_CONDUCT.md

[martijnrusschen]

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[codecov]

Codecov Report

Merging #1190 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1190   +/-   ##
======================================
  Coverage    85.3%   85.3%           
======================================
  Files          13      13           
  Lines         898     898           
  Branches      149     149           
======================================
  Hits          766     766           
  Misses         43      43           
  Partials       89      89

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