upload: add suggestion on how to upload a cover for a video

cds/modules/deposit/static/templates/cds_deposit/types/video/uploader.html

@@ -124,6 +124,14 @@
       </div>
     </div>
   </div>
+  <hr class="my-10" />
+  <div class="pa-10 text-muted">
+    <h5 class="text-muted"><strong>Tips and suggestions</strong></h5>
+    <ul>
+      <li>Want to customize the cover of your video? Just upload an image named <mark>poster.png</mark> or <mark>poster.jpg</mark>. Need to update it later? Simply upload a new image with the same name, and the cover will be replaced automatically.
+      </li>
+    </ul>
+  </div>
 </div>
 <!-- Thumbnail -->
 <!-- Subtitles -->