chore(docs) Add a note about gridOptions.footerTemplate

misc/tutorial/105_footer.ngdoc

@@ -11,6 +11,9 @@ You can also pass in a function in order to create your own aggregation logic.
 If you set the `aggregationHideLabel` option on the columnDef to true, then the column will
 show the aggregation but without a label.  Refer the third column in the example below.
 
+<br/>
+You can override the default grid footer template with gridOptions.footerTemplate.  Copy the default ui-grid-footer.html from the source as your starting point.  <i>This is not demonstrated in this tutorial.</i>
+
 @example
 <example module="app">
   <file name="app.js">