PULL_REQUEST_TEMPLATE: Add "Notify maintainers" section

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.github
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.github/PULL_REQUEST_TEMPLATE.md
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- [ ] Ensured that relevant documentation is up to date
- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
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---
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###### Notify maintainers
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cc @