Added examples for SectionList (#30390)

Summary:
`Added examples for the following missing props in SectionList`
* Prop: initialNumToRender
* Prop: ListEmptyComponent
* Prop: onEndReached

**Motivation** - Missing examples for these props in the RNTester app
## Changelog

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[General] [Added] - `Added examples for the missing props in SectionList`

Pull Request resolved: https://github.com/facebook/react-native/pull/30390

Test Plan:
![empty](https://user-images.githubusercontent.com/26821140/99139935-49667000-2663-11eb-8b89-5a6b17abc3e9.png)
![end](https://user-images.githubusercontent.com/26821140/99139944-4e2b2400-2663-11eb-8e95-08e863614dec.png)

Reviewed By: cpojer

Differential Revision: D25245322

Pulled By: rickhanlonii

fbshipit-source-id: 3e44b9cf86128f1d3df535799519631cf2296a7e

packages/rn-tester/js/examples/SectionList/SectionListExample.js

@@ -67,6 +67,12 @@ const CustomSeparatorComponent = ({highlighted, text}) => (
   </View>
 );
 
+const EmptySectionList = () => (
+  <View style={{alignItems: 'center'}}>
+    <Text style={{fontSize: 20}}>This is rendered when the list is empty</Text>
+  </View>
+);
+
 class SectionListExample extends React.PureComponent<{...}, $FlowFixMeState> {
   state:
     | any
@@ -174,6 +180,15 @@ class SectionListExample extends React.PureComponent<{...}, $FlowFixMeState> {
           renderSectionHeader={renderSectionHeader}
           renderSectionFooter={renderSectionFooter}
           stickySectionHeadersEnabled
+          initialNumToRender={10}
+          ListEmptyComponent={EmptySectionList}
+          onEndReached={() =>
+            Alert.alert(
+              'onEndReached called',
+              'You have reached the end of this list',
+            )
+          }
+          onEndReachedThreshold={0}
           sections={[
             {
               key: 'empty section',