Add getSubtitle to typings and docs (react-native-push-notification#413)

Author: fiznool

README.md

@@ -762,6 +762,16 @@ Gets the notification's title from the `aps` object
 
 ---
 
+### `getSubtitle()`
+
+```jsx
+getSubtitle();
+```
+
+Gets the notification's subtitle from the `aps` object
+
+---
+
 ### `getContentAvailable()`
 
 ```jsx

index.d.ts

@@ -97,6 +97,11 @@ export interface PushNotification {
    */
   getTitle(): string;
 
+  /**
+   * Gets the notification's subtitle from the `aps` object
+   */
+  getSubtitle(): string | undefined;
+
   /**
    * Gets the content-available number from the `aps` object
    */