PULL_REQUEST_TEMPLATE.md: Request PRs describe themselves

There are a lot of PRs for updates that don't make it easy to find
out what changes might be breaking and lots of PRs for new packages
that don't describe what the new packages is or does.

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,8 +8,12 @@ List of open PRs: https://github.com/NixOS/nixpkgs/pulls
 Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-contributions
 -->
 
-###### Motivation for this change
+###### Description of changes
 
+<!--
+For package updates please link to a changelog or describe changes, this helps your fellow maintainers discover breaking updates.
+For new packages please briefly describe the package or provide a link to its homepage.
+-->
 
 ###### Things done