Add setup-detekt action to README

[cortinico]

See #3369 for reference.

[codecov]

Codecov Report

Merging #3373 (1ab790e) into master (c87a0ac) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3373   +/-   ##
=========================================
  Coverage     80.31%   80.31%           
  Complexity     2721     2721           
=========================================
  Files           445      445           
  Lines          8248     8248           
  Branches       1566     1566           
=========================================
  Hits           6624     6624           
  Misses          785      785           
  Partials        839      839           

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