README: add use case for write_to
README: add use case for write_to

Ref: https://github.com/pypa/setuptools_scm/issues/273#issuecomment-401555022

[ci skip]

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