docs(readme): add codecov badge

[codecov[bot]]

Codecov Report

Merging #3 (2278c84) into master (1a9261f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files           7        7           
  Lines         659      659           
=======================================
  Hits          637      637           
  Misses         18       18           
  Partials        4        4           

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[github-actions[bot]]

:tada: This PR is included in version 0.2.0 :tada:

The release is available on GitHub release

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