doc(README.md): add badges

[bassosimone]

This diff adds build status and coverage badges to the toplevel README.md.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.25%. Comparing base (d0dd721) to head (5c821e9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   80.25%   80.25%           
=======================================
  Files          21       21           
  Lines        2046     2046           
=======================================
  Hits         1642     1642           
  Misses        404      404           

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