Add steps on how to run cookiecutter tests

[sfmig]

Is your feature request related to a problem? Please describe.
Because the cookiecutter repo is not a fully fledged Python package, it can be confusing for contributors to understand how to run the tests for the project.

Describe the solution you'd like
Clear steps on how to run the cookiecutter project tests. This could be in the README, or maybe in a newly added CONTRIBUTING.md file.

Describe alternatives you've considered
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Additional context
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[sfmig]

This could also be added to the cookiecutter docs.

I found the test dependencies are described in commented lines here, which is not the best.