docs: update PR template for CHANGELOG reminder

[shaneutt]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

While working on #1183 I found it was quite laborious to search all PR history and fill in the CHANGELOG.md retroactively.

This PR is a proposal instead to indicate that user-facing changes must be accompanied by a CHANGELOG.md update in the PR they are introduced, adding a PR checklist to the PR template. Consequently, this also proposes that we keep a heading of Unreleased in the CHANGELOG as a place for contributors to place their entries prior to release to reduce confusion about where it should go.

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[robscott]

Thanks @shaneutt! I like this idea. I'm a bit concerned that we may end up with a lot of merge conflicts on CHANGELOG.md if we take this approach, but it could be worth trying to see if that really is an issue.

[robscott]

Maybe add some kind of note here to indicate that unreleased changes may be rolled back or changed prior to a release.

[shaneutt]

Sounds good. Change in 1e06e53

[robscott]

If we're doing this, we probably want to remove the release-note section above

[shaneutt]

👍 Changed in 1e06e53

[shaneutt]

@youngnick any feelings on this one?

[youngnick]

In Contour, we use a changelogs directory, with an unreleased directory below that, and each PR has to include a separate .md file with the changelog in it. This avoids conflicts in a singular changelog file, but makes the release process pretty easy. Ideally, we can make it so your PR won't go green without the changelog present (in Contour we use Actions, but we'd need to figure out a way to add this to Prow's build, I think).

Edit: I think that the current idea is okay for now though.

[shaneutt]

Alright, sounds like so far people are OK with trying this out. I'll bring it up at monday's meeting too, if it's not otherwise merged before then. Thank you.

[shaneutt]

@youngnick, @robscott just wanted to ping on this one because it's been sitting a while: think we're OK to give this one a try?

[robscott]

Actually before we go too far down this path, maybe we should try the upstream scripting for release notes generation: https://github.com/kubernetes/release/blob/master/cmd/release-notes/README.md.

[shaneutt]

Alright, that goes a bit beyond the scope of what I was trying to accomplish here, so I'll consider this one closed for the time being and then we should probably create a follow up issue for that 👍