Add a tooltip to document the Settle button

ihatemoney/templates/settle_bills.html

@@ -19,7 +19,9 @@
             <td>
                 <span id="settle-bill" class="ml-auto pb-2">
                     <a href="{{ url_for('.settle', amount = bill.amount, ower_id = bill.ower.id, payer_id = bill.receiver.id) }}" class="btn btn-primary">
+                      <div data-toggle="tooltip" title='{{ _("Click here to record that the money transfer has been done") }}'>
                         {{ ("Settle") }}
+                      </div>
                     </a>
                 </span>
             </td>