Add and document the project_urls field.

[phildini]

This field corresponds to
https://packaging.python.org/specifications/core-metadata/#project-url-multiple-use
and better documentation in the sampleproject will hopefully encourage use.

This PR comes by way of pypa/setuptools#1276.

[phildini]

If this seems to be in the right direction, I will file a matching PR in https://github.com/pypa/python-packaging-user-guide to more thoroughly document.

[di]

One thing to note here is that the key is what's used to render the link text on PyPI. I think it would be ideal to make it clear to a user that they can make this whatever they want, by making the examples looks more human-friendly. It'd also be great to include some of the more common project_urls that this is used for, maybe something like:

    project_urls={  # Optional
        'Bug Reports': 'https://github.com/pypa/sampleproject/issues',
        'Source': 'https://github.com/pypa/sampleproject/',
        'Funding': 'https://donate.pypi.org',
        'Say Thanks!': 'http://saythanks.io/to/example'
    },

If I understand correctly, you're hoping for some consistency with the project keys here to find projects that support saying thanks, but since it's really not enforceable, a better approach might be to just look at all the project_urls values and see if any match what you're looking for.

[phildini]

@di I can definitely add more links! I agree that these aren't enforceable, but I'm hoping good docs and usage patterns will encourage some consistency.

[di]

Thanks @phildini!