Document `python -m setuptools_scm` for debugging
Document `python -m setuptools_scm` for debugging

This change adds a little text in the README file mentioning how the
`python -m setuptools_scm` command can be used to help debugging
configurations and previewing the generated version string.

pypa/setuptools_scm,RonnyPfannschmidt/setuptools_scm,RonnyPfannschmidt/setuptools_scm,pypa/setuptools_scm