Reduce checkboxes in PR checklist (#5120)

The basics around how to format commits and PR settings are primarily
relevant when opening a PR for the first time. But for repeated
contributors it is tedious to always tick those checkboxes.

Co-authored-by: rawtaz <rawtaz@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -28,13 +28,15 @@ Checklist
 You do not need to check all the boxes below all at once. Feel free to take
 your time and add more commits. If you're done and ready for review, please
 check the last box. Enable a checkbox by replacing [ ] with [x].
+
+Please always follow these steps:
+- Read the [contribution guidelines](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#providing-patches).
+- Enable [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).
+- Run `gofmt` on the code in all commits.
+- Format all commit messages in the same style as [the other commits in the repository](https://github.com/restic/restic/blob/master/CONTRIBUTING.md#git-commits).
 -->
 
-- [ ] 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).
 - [ ] I have added tests for all code changes.
 - [ ] I have added documentation for relevant 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.