Local upload settings setup.cfg are not passed to twine anymore

[wosc]

After #309 / 6.18.0 zest.releaser does not read upload repository configuration from a local setup.cfg to pass on to twine anymore.

You can work around this by setting TWINE_* environment variables, but there is now some dissonance, because the setup.cfg is still needed to tell zest.releaser which index-servers to use, or rather mostly, to not employ its "default pypi" support mechanics.

I'm not sure what a nicer support system could look like, but at the least the current state of affairs should be mentioned in the documentation, I guess.