Update README.md

documentation(DOC-3343): KB link

Author: 9629976110

README.md

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 This example illustrates how to define summary rows using AttachedProperty in [WPF DataGrid](https://www.syncfusion.com/wpf-ui-controls/datagrid) (SfDataGrid).
 
 [WPF DataGrid](https://www.syncfusion.com/wpf-ui-controls/datagrid) (SfDataGrid) provides support to show the column summary. If the DataContext of SfDataGrid is ViewModel, you can bind [SummaryColumns](http://help.syncfusion.com/cr/cref_files/wpf/Syncfusion.SfGrid.WPF~Syncfusion.UI.Xaml.Grid.GridSummaryRow~SummaryColumns.html) to a property in ViewModel using the [AttachedProperty](https://docs.microsoft.com/en-us/dotnet/framework/wpf/advanced/attached-properties-overview) of type List<GridSummaryColumn>.
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+KB article - [How to define summary rows using AttachedProperty in WPF and UWP DataGrid (SfDataGrid)](https://www.syncfusion.com/kb/9838/how-to-define-summary-rows-using-attached-property-in-wpf-datagrid-sfdatagrid)