Update README

[sh-cho]

• Fix unit test workflow badge
  (ref: Change to GitHub workflow badge routes badges/shields#8671)
• Add LICENSE link to file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.35%. Comparing base (c5e730e) to head (b32ffa4).
Report is 1 commits behind head on master.

❗ Current head b32ffa4 differs from pull request most recent head e6155c8. Consider uploading reports for the commit e6155c8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   75.94%   75.35%   -0.59%     
==========================================
  Files           7        7              
  Lines        1193     1193              
==========================================
- Hits          906      899       -7     
- Misses        196      202       +6     
- Partials       91       92       +1     

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