restic/.github/PULL_REQUEST_TEMPLATE.md

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What changes are made in this PR? What problem does it solve?
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Checklist
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- [ ] I have read the [contribution guidelines](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#providing-patches).
- [ ] I have [enabled maintainer edits](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork) for this PR.
- [ ] I have added tests for all code changes in this PR.
- [ ] I have added documentation for the those changes (in the manual).
- [ ] There's a new file in `changelog/unreleased/` that describes the changes for our users (see [template](https://github.com/restic/restic/blob/master/changelog/TEMPLATE)).
- [ ] I have run `gofmt` on the code in all commits.
- [ ] All commit messages are formatted in the same style as [the other commits in the repo](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#git-commits).
- [ ] I'm done! This pull request is ready for review.