Add contributor usernames to changelog

[bhrutledge]

In the process of switching to towncrier, I updated the contributing/maintainer guide to favor ease of adding changelog entries and building the final changelog. That includes the guideline that "you don’t need to add your name to the entry, since that will be associated with the pull request." In surveying other projects that use towncrier, I didn't see a clear pattern of including usernames, so I opted to leave them out.

However, I can see how it would be friendlier to contributors to feature them in the changelog, and it looks like virtualenv and tox are consistent about that. Unless other maintainers advocate for a different approach, I'll aim to review some of their PR's to see how that works in practice, and follow-up with a PR that updates Twine's contributing guide and retroactively adds PR authors to some recent releases.

Thanks @brainwane for the suggestion.

[sigmavirus24]

🤷🏽 I don't feel strongly one way or another. Either way, the update you made was important - this should all be optional.

[jaraco]

In my opinion, Github provides sufficient attribution, so I try to avoid imposing additional manual steps. In Setuptools, the other contributors felt it important enough to add it as a recommendation, but I'm with Ian on this - let's not require additional steps.

[bhrutledge]

Closing as a duplicate of #648.