Add a comment noting how these checklists are intended to be used.

Lots of folks have been checking the boxes here when they submit the
template, however my intention was that they would populate the
incomplete checklist and the ros2-gbp admins would check those off when
they import the repo. We'll see if this comment improves the situation
or not. If it doesn't I may remove the checklist from the template
entirely.

.github/ISSUE_TEMPLATE/new_release_repository.md

@@ -7,6 +7,10 @@ title: 'Add new release repositories'
 * Name of the release team: 
 
 * Release repositories to add:
+  <!--
+  If there are any existing release repositories which should have their contents imported into the ros2-gbp organization, list and link to them here.
+  Leave the checkbox _unchecked_. The ros2-gbp administrator will check the box when they have completed the repository import.
+  -->
   * [FIRST_REPOSITORY](SOURCE URL)
     * [ ] There is an existing release repository which should be imported: <RELEASE REPOSITORY URL>
   * [SECOND REPOSITORY](SOURCE URL)