Added placeholder attributes example to RNTester (#23769)

Summary:
After some works, we already support some attributes to text input's placeholder, so we can add example to show it in RNTester.

[iOS] [Added] - Added placeholder attributes example to RNTester
Pull Request resolved: https://github.com/facebook/react-native/pull/23769

Differential Revision: D14436086

Pulled By: cpojer

fbshipit-source-id: 8637dcb82a2e6aa51e351fd696b049aaee893997

RNTester/js/TextInputExample.ios.js

@@ -530,6 +530,11 @@ const styles = StyleSheet.create({
     padding: 4,
     marginBottom: 4,
   },
+  multilinePlaceholderStyles: {
+    letterSpacing: 10,
+    lineHeight: 20,
+    textAlign: 'center',
+  },
   multilineExpandable: {
     height: 'auto',
     maxHeight: 100,
@@ -541,6 +546,10 @@ const styles = StyleSheet.create({
     fontFamily: 'Cochin',
     height: 60,
   },
+  singlelinePlaceholderStyles: {
+    letterSpacing: 10,
+    textAlign: 'center',
+  },
   eventLabel: {
     margin: 3,
     fontSize: 12,
@@ -1124,4 +1133,26 @@ exports.examples = [
       );
     },
   },
+  {
+    title: 'TextInput Placeholder Styles',
+    render: function() {
+      return (
+        <View>
+          <WithLabel label="letterSpacing: 10 lineHeight: 20 textAlign: 'center'">
+            <TextInput
+              placeholder="multiline text input"
+              multiline={true}
+              style={[styles.multiline, styles.multilinePlaceholderStyles]}
+            />
+          </WithLabel>
+          <WithLabel label="letterSpacing: 10 textAlign: 'center'">
+            <TextInput
+              placeholder="singleline"
+              style={[styles.default, styles.singlelinePlaceholderStyles]}
+            />
+          </WithLabel>
+        </View>
+      );
+    },
+  },
 ];