Add ros/rosdistro source field links to issue template.

As discusesed, we want repositories to undergo naming and license review
as source packages before we create release repositories for them.

Right now this process is quite manual but building tools which
streamline the workflow is planned.

.github/ISSUE_TEMPLATE/new_release_repository.md

@@ -12,7 +12,9 @@ title: 'Add new release repositories'
   Leave the checkbox _unchecked_. The ros2-gbp administrator will check the box when they have completed the repository import.
   -->
   * [FIRST_REPOSITORY](SOURCE URL)
+    * [ros/rosdistro source entry](LINK TO SOURCE FIELD IN ROS/ROSDISTRO)
     * [ ] There is an existing release repository which should be imported: <RELEASE REPOSITORY URL>
   * [SECOND REPOSITORY](SOURCE URL)
+    * [ros/rosdistro source entry](LINK TO SOURCE FIELD IN ROS/ROSDISTRO)
     * [ ] There is an existing release repository which should be imported: <RELEASE REPOSITORY URL>
   * ...