doc: release badge in README

[fredbi]

Also: pinned cache action (fuzz test job)

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.00%. Comparing base (58fdaa4) to head (412c4a3).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   79.00%   79.00%           
=======================================
  Files           2        2           
  Lines         281      281           
=======================================
  Hits          222      222           
  Misses         40       40           
  Partials       19       19           

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