github/PULL_REQUEST_TEMPLATE: replace first heading with comment

When creating a PR from a branch that only adds a single commit, this heading
would always jankily be left *below* the actual commit message because github
simply inserts the commit message before the template.

The heading also only served as a light call to action whereas the comment is
rather explicit in asking the PR author to provide a proper PR description.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,7 @@
-## Description of changes
 
 <!--
+^ Please summarise the changes you have done and explain why they are necessary here ^
+
 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.
 -->