PULL_REQUEST_TEMPLATE: move call-for-review to the end

This improves URL previews like the ones on Discourse, where currently
the boilerplate comment is printed for every PR link instead of parts of
the motivation.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1,3 @@
-<!--
-To help with the large amounts of pull requests, we would appreciate your
-reviews of other pull requests, especially simple package updates. Just leave a
-comment describing what you have tested in the relevant package/service.
-Reviewing helps to reduce the average time-to-merge for everyone.
-Thanks a lot if you do!
-List of open PRs: https://github.com/NixOS/nixpkgs/pulls
-Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-contributions
--->
-
 ###### Description of changes
 
 <!--
@@ -38,3 +28,14 @@ For new packages please briefly describe the package or provide a link to its ho
   - [ ] (Module addition) Added a release notes entry if adding a new NixOS module
   - [ ] (Release notes changes) Ran `nixos/doc/manual/md-to-db.sh` to update generated release notes
 - [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md).
+
+<!--
+To help with the large amounts of pull requests, we would appreciate your
+reviews of other pull requests, especially simple package updates. Just leave a
+comment describing what you have tested in the relevant package/service.
+Reviewing helps to reduce the average time-to-merge for everyone.
+Thanks a lot if you do!
+
+List of open PRs: https://github.com/NixOS/nixpkgs/pulls
+Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-contributions
+-->