Add Codecov badge

[gdalle]

Add Codecov badge to README

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (47a7ec6) to head (8ad0b62).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #42 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 6 6 Lines 63 63 ========================================= Hits 63 63 ```

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