Update README.md (#3395)

* Update README.md

Updated documentation for a common issue, (Highly Suggest placing this at the to of the MapView Component Documentation, but I will follow community standards for now)

* Update README.md

Updated to be more descriptive.

README.md

@@ -652,6 +652,29 @@ Good:
 </View>
 ```
 
+### Children Components Not Re-Rendering
+Components that aren't declared by this library (Ex: Markers, Polyline) must not be children of the MapView component due to MapView's unique rendering methodology. Have your custom components / views outside the MapView component and position absolute to ensure they only re-render as needed.
+Example:
+Bad:
+
+```jsx
+  <View style={StyleSheet.absoluteFillObject}>
+    <MapView style={StyleSheet.absoluteFillObject}>
+      <View style={{ position: 'absolute', top: 100, left: 50 }}/>
+    </MapView>
+  </View>
+```
+
+Good:
+
+```jsx
+  <View style={StyleSheet.absoluteFillObject}>
+    <MapView style={StyleSheet.absoluteFillObject} />
+    <View style={{ position: 'absolute', top: 100, left: 50 }}/>
+  </View>
+```
+
+Source: https://github.com/react-native-community/react-native-maps/issues/1901
 
 License
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