add PR template

[mr-tz]

in support of #457

[Ana06]

thanks for taking care of this @mr-tz!

[Ana06]

Should we set up the GitHub action and a templete in the CHANGELOG to ensure this is done before adding it here? I was thinking about setting this up together with the next release to be able to start with a empty CHANGELOG.

In #457 I suggested to use a label if no CHANGELOG changes are needed. But contributors don't have permissions for that. Maybe a section like the one for the tests would be better, so that we can add the label if no changes are needed. Something like:

### Changelog

- [ ] I have updated the [CHANGELOG.md](/CHANGELOG.md)
- [ ] The [CHANGELOG.md](/CHANGELOG.md) doesn't need to be updated (changelog linter will fail until a maintainer adds the `no changelog` label)

[mr-tz]

Should we set up the GitHub action and a templete in the CHANGELOG to ensure this is done before adding it here?

1. Adding the template is a good idea, I think the Action is optional for now.

2. Wouldn't any change require a respective CHANGELOG entry? Let's try to keep it simple.

[Ana06]

@mr-tz

Wouldn't any change require a respective CHANGELOG entry? Let's try to keep it simple.

We could give it a try. 🤔

[Ana06]

Checklists in PR sound like a good idea, but my experience is that in practice they are just removed because they are annoying to fill. I am up to give it a try, but I am not sure if we need a checklist for the type of change if soon it is expected that the author updates the changelog (which has this information).

[mr-tz]

I like this part to get a quick overview of what the PR provides. I've removed the actual Checklist entry.