###### Things done

- [ ] Tested using sandboxing
  ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,
    or option `build-use-sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
    on non-NixOS)
- Built on platform(s)
   - [ ] NixOS
   - [ ] OS X
   - [ ] Linux
- [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nox --run "nox-review wip"`
- [ ] Tested execution of all binary files (usually in `./result/bin/`)
- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).

---


###### Motivation for this change


###### Things done

- [ ] Tested using sandboxing
  ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,
    or option `build-use-sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
    on non-NixOS)
- Built on platform(s)
   - [ ] NixOS
   - [ ] OS X
   - [ ] Linux
- [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nox --run "nox-review wip"`
- [ ] Tested execution of all binary files (usually in `./result/bin/`)
- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).

---


Update PR template with motivation for changes
Update PR template with motivation for changes

I'm seeing a lot of PRs that only include the checklist, when the main thing I care about when I'm reviewing a PR is what the author was thinking when they decided to make the change.