Document the "Do not upload" classifier trick

[pganssle]

In this tweet, @di mentions a trick to prevent accidentally uploading your packages to PyPI by adding a Private :: Do Not Upload classifier and in the thread @ewdurbin mentioned that he had made Private a reserved top-level classifier so the trick works reliably.

It seems like it would be a good idea to document this somewhere for easy linking. It could go into a tutorial or maybe we can add a "miscellaneous tips and tricks" section?

[ewdurbin]

https://github.com/pypa/warehouse/blob/master/warehouse/templates/pages/classifiers.html which renders https://pypi.org/classifiers/ could probably also address this.

[Julian]

Should there be a License :: Closed Source for this? Having two ways to do it isn't great obviously, but possibly the advantage of using License is some tools maybe care about being able to ask about the license anyhow from classifiers, so maybe using that namespace solves both?

[pfmoore]

I may well have an open source project that I don't want to upload to PyPI (maybe because it's not yet ready for broad distribution).

[imankulov]

I came across this issue while finding a way to mark my python packages as private. Adding a cross-reference to a change in Warehouse which sort of "institutionalizes the trick": pypi/warehouse#5440

[hugovk]

https://github.com/pypa/warehouse/blob/master/warehouse/templates/pages/classifiers.html which renders pypi.org/classifiers could probably also address this.

Please see PR pypi/warehouse#10872 to document it on that page.

Is that sufficient or is something also needed on https://packaging.python.org?

[hugovk]

Is that sufficient or is something also needed on packaging.python.org?

Please see #1056 as well :)