Add minor Docs.

doc/03_Grid.md

@@ -11,3 +11,12 @@ A column configuration represents how the column should behave, for example if i
 
 ## ColumnData
 A column data is the actual data for a column. It has a reference to the column and the actual data.
+
+
+## Filter
+A filter is a way to filter the data in the grid. One Property of getting the grid data is the `filter` property.
+Here you can define `page`, `pageSize` and `includeDescendants`.
+
+`page` is the page number of the data you want to get. 
+`pageSize` is the number of items you want to get. `
+includeDescendants` is a boolean value to include the descendants of the current item.