Add reminder to provide receipts.

index.html

@@ -105,6 +105,11 @@
 
     <h2 id="suggest">Suggest instance</h2>
 
+    <p>
+      Please make sure to include receipts (hyperlinks or screenshots) for all claims you make. We
+      don't want trust to be necessary to use this list.
+    </p>
+
     <form method="post" action="/add" enctype="multipart/form-data">
       <label for="instance">Instance:</label>
       <input type="text" id="instance" name="instance" placeholder="example.com" autofocus required>