Add very basic usage example for android toast

Summary:
Currently, it's hard to find out that it's very easy to use android toasts. The example is very complex and hard to grasp for a beginner. So, I propose to add an easy to understand example.

I'm sure the syntax of this PR is not really correct, it's just a start to show the kind of thing I propose.
Closes https://github.com/facebook/react-native/pull/9859

Differential Revision: D3886274

Pulled By: donyu

fbshipit-source-id: 15e693f5ddb1efea0fc6b7accfa688fd5f99a100

Libraries/Components/ToastAndroid/ToastAndroid.android.js

@@ -21,7 +21,13 @@ var RCTToastAndroid = require('NativeModules').ToastAndroid;
  * 2. int duration: The duration of the toast. May be ToastAndroid.SHORT or ToastAndroid.LONG
  *
  * There is also a function `showWithGravity` to specify the layout gravity. May be
- * ToastAndroid.TOP, ToastAndroid.BOTTOM, ToastAndroid.CENTER
+ * ToastAndroid.TOP, ToastAndroid.BOTTOM, ToastAndroid.CENTER.
+ *
+ * Basic usage:
+ * ```javascript
+ * ToastAndroid.show('A pikachu appeared nearby !', ToastAndroid.SHORT);
+ * ToastAndroid.showWithGravity('All Your Base Are Belong To Us', ToastAndroid.SHORT, ToastAndroid.CENTER);
+ * ```
  */
 
 var ToastAndroid = {