feat: add badge for code coverage (#722)

Author: cyrraz

README.md

@@ -149,14 +149,15 @@ backports structure with a small typing example.
     - Replaces Flake8, isort, pyupgrade, yesqa, pycln, and dozens of plugins
   - Includes spell checking
 - One of several popular licenses
-- An pylint nox target can be used to run pylint, which integrated GHA
+- A pylint nox target can be used to run pylint, which integrated GHA
   annotations
 - A ReadTheDocs-ready Sphinx or MkDocs `docs/` folder and `docs`
   dependency-group
 - A `tests/` folder and pytest `test` dependency-group
 - A dev group for `uv run` integration
 - A noxfile is included with a few common targets
 - A README
+- Code coverage reporting with automatic uploads to Codecov after tests run
 
 #### For developers:
 

{{cookiecutter.project_name}}/README.md

@@ -11,6 +11,8 @@
 {%- if cookiecutter.org | lower == "scikit-hep" %}
 [![Scikit-HEP][sk-badge]](https://scikit-hep.org/) {%- endif %}
 
+[![Coverage][coverage-badge]][coverage-link]
+
 {%- if cookiecutter.docs == "sphinx" %}
 
 <!-- SPHINX-START -->
@@ -31,5 +33,7 @@
 {%- if cookiecutter.org | lower == "scikit-hep" %}
 [sk-badge]:                 https://scikit-hep.org/assets/images/Scikit--HEP-Project-blue.svg
 {%- endif %}
+[coverage-badge]:           https://codecov.io/{{cookiecutter.__ci}}/{{cookiecutter.org}}/{{cookiecutter.project_name}}/branch/main/graph/badge.svg
+[coverage-link]:            https://codecov.io/{{cookiecutter.__ci}}/{{cookiecutter.org}}/{{cookiecutter.project_name}}
 
 <!-- prettier-ignore-end -->