doc: Add a new usage example for CellMeasurer

docs/CellMeasurer.md

@@ -25,8 +25,8 @@ It should be configured based on the type of measurements you need. It accepts t
 ### Prop Types
 | Property | Type | Required? | Description |
 |:---|:---|:---:|:---|
-| defaultHeight | number | | Umeasured cells will initially report this height |  
-| defaultWidth | number | | Umeasured cells will initially report this width |
+| defaultHeight | number | | Unmeasured cells will initially report this height |  
+| defaultWidth | number | | Unmeasured cells will initially report this width |
 | fixedHeight | boolean | | Rendered cells will have a fixed height, dynamic width |
 | fixedWidth | boolean | | Rendered cells will have a fixed width, dynamic height |
 | minHeight | number | | Derived row height (of multiple cells) should not be less than this value |
@@ -162,6 +162,20 @@ function renderList (props) {
 }
 ```
 
+###### Using `CellMeasurer` with equal dynamic cell heights or widths
+
+```jsx
+// Normally, every cell gets measured individually and is very slow.
+// However, with the keyMapper prop we can specify a constant return value and
+// tell CellMeasurer that all measurements after the first one will hit the
+// cache and we get a speedy solution.
+const cache = new CellMeasurerCache({
+  defaultHeight: 30,
+  fixedWidth: true,
+  keyMapper: () => 1,
+});
+```
+
 ### Limitations and Performance Considerations
 
 ###### Performance