packages: add note about multi-selection in stale relations

templates/packages/stale_relations.html

@@ -7,6 +7,7 @@
 {% block content %}
 <div class="box">
     <h2>Stale Package Relations</h2>
+    <i>Select multiple rows by clicking on a checkbox and holding down they shift key.</i>
 
     <form id="stale-relations-form" method="post" action="update/">{% csrf_token %}
     <h3>Inactive User Relations ({{ inactive_user|length }})</h3>