add code coverage badge

[xyb]

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[codecov-commenter]

Codecov Report

Merging #26 (9aa7a2e) into main (e333d6f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files           6        6           
  Lines         700      700           
=======================================
  Hits          632      632           
  Misses         68       68           

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