Add help text for footnote value.

js/plugins/reference_footnotes/dialogs/footnotes.js

@@ -61,6 +61,12 @@
                                     if (isEdit)
                                         this.setValue(element.getAttribute('value'));
                                 }
+
+                            },
+                          {
+                            id: 'value-help',
+                            type: 'html',
+                            html: 'Leave blank for an automatic sequential reference number, or enter a custom footnote value',
                           }
                         ],
                 }