Put the GitLab YAML in a file instead of directly in the README

[codecov[bot]]

Codecov Report

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

Project coverage is 99.50%. Comparing base (0128ace) to head (d5336f8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #489 +/- ## =========================================== - Coverage 100.00% 99.50% -0.50% =========================================== Files 12 12 Lines 405 405 =========================================== - Hits 405 403 -2 - Misses 0 2 +2 ```

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