Add Codecov badge to README

README.rst

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 Unit definitions for integrated-assessment research
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+|pypi| |gha| |coverage|
 
 .. |pypi| image:: https://img.shields.io/pypi/v/iam-units.svg
    :target: https://pypi.python.org/pypi/iam-units/
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    :target: https://github.com/IAMconsortium/units/actions/workflows/test.yaml
    :alt: Build status
 
+.. |coverage| image:: https://codecov.io/gh/IAMconsortium/units/graph/badge.svg?token=L0H171CF9O
+   :target: https://codecov.io/gh/IAMconsortium/units
+   :alt: Test coverage
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 © 2020–2024 `IAM-units authors`_; licensed under the `GNU GPL version 3`_.
 
 The file `definitions.txt`_ gives `Pint`_-compatible definitions of energy, climate, and related units to supplement the SI and other units included in Pint's `default_en.txt`_.