.github: move text in the PR template into a comment

The majority of pull requests do not fix a downstream issue. In
practice, people usually delete the whole suggested content.

Move it into a comment, so that this step is unnecessary.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,8 @@
-# Description
+<!--
+Thanks for proposing a change to osbuild-composer!
 
-
-
-**Downstream testing notes:**
+If this pull request fixes a downstream issue, please refer to test/README.md
+and uncomment this checklist:
 
 - [ ] 1st commit of any `rhbz#` related PR contains bug reproducer; CI reports FAIL or
 - [ ] PR contains automated tests for new functionality and
@@ -10,4 +10,4 @@
 - [ ] Subsequent commits provide bug fixes without modifying the reproducer; CI reports PASS and
 - [ ] QE approves this PR; RHBZ status is set to `MODIFIED + Verified=Tested`
 
-For more info see [test/README.md](../test/README.md#downstream-testing-notes)!
+-->