doc: added note for forceUpdateGrids for MultiGrid in README

Author: fluke

README.md

@@ -180,7 +180,7 @@ For example:
 ###### Public methods
 
 `Grid` and `Collection` components can be forcefully re-rendered using [`forceUpdate`](https://facebook.github.io/react/docs/component-api.html#forceupdate).
-For `Table` and `List`, you'll need to call [`forceUpdateGrid`](https://github.com/bvaughn/react-virtualized/blob/master/docs/Table.md#forceupdategrid) to ensure that the inner `Grid` is also updated.
+For `Table` and `List`, you'll need to call [`forceUpdateGrid`](https://github.com/bvaughn/react-virtualized/blob/master/docs/Table.md#forceupdategrid) to ensure that the inner `Grid` is also updated. For `MultiGrid`, you'll need to call [`forceUpdateGrids`](https://github.com/bvaughn/react-virtualized/blob/master/docs/MultiGrid.md#forceupdategrids) to ensure that the inner `Grid`s are updated.
 
 Documentation
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