Add code of conduct markdown

[pnoltes]

Small PR that introduces a code of conduct document.

The document simply refers to the code of conduct of the ASF. When reviewing, also read the referenced document to check if you agree with the code of conduct.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 91.52%. Comparing base (c6d0e2a) to head (c8dd7c4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #809      +/-   ##
==========================================
+ Coverage   91.44%   91.52%   +0.07%     
==========================================
  Files         234      234              
  Lines       28655    28655              
==========================================
+ Hits        26204    26226      +22     
+ Misses       2451     2429      -22     

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