CI: Add README coverage badge

[ueco-jb]

Experiment using grcov instead of tarpaulin and uploading the results to codecov.
Add coverage readme badge.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@183101b). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head ade6577 differs from pull request most recent head d87b28d. Consider uploading reports for the commit d87b28d to get more accurate results

Additional details and impacted files

@@         Coverage Diff          @@
##             main   #64   +/-   ##
====================================
  Coverage        ?     0           
====================================
  Files           ?     0           
  Lines           ?     0           
  Branches        ?     0           
====================================
  Hits            ?     0           
  Misses          ?     0           
  Partials        ?     0           

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