adds Keyboard.dismiss() function and document Keyboard

Summary:
Simple and elegant.  Now someone can dismiss a keyboard in a way that makes sense.

```js
import { Keyboard } from 'react-native'

// Hide that keyboard!
Keyboard.dismiss()
```

+ docs
Closes https://github.com/facebook/react-native/pull/9925

Differential Revision: D3935357

fbshipit-source-id: ecd2fb5c72c4dd769951d308e9bb6ee5d888052a

Libraries/Components/Keyboard/Keyboard.js

@@ -13,5 +13,99 @@
 
 const NativeEventEmitter = require('NativeEventEmitter');
 const KeyboardObserver = require('NativeModules').KeyboardObserver;
+const dismissKeyboard = require('dismissKeyboard');
+const KeyboardEventEmitter = new NativeEventEmitter(KeyboardObserver);
 
-module.exports = new NativeEventEmitter(KeyboardObserver);
+// The following object exists for documentation purposes
+// Actual work happens in
+// https://github.com/facebook/react-native/blob/master/Libraries/EventEmitter/NativeEventEmitter.js
+/**
+ * `Keyboard` component to control keyboard events.
+ *
+ * ### Usage
+ *
+ * The Keyboard component allows you to listen for native events and react to them, as
+ * well as make changes to the keyboard, like dismissing it.
+ *
+ *```
+ * import React, { Component } from 'react';
+ * import { Keyboard, TextInput } from 'react-native';
+ *
+ * class Example extends Component {
+ *   componentWillMount () {
+ *     this.keyboardDidShowListener = Keyboard.addListener('keyboardDidShow', this._keyboardDidShow);
+ *     this.keyboardDidHideListener = Keyboard.addListener('keyboardDidHide', this._keyboardDidHide);
+ *   }
+ *
+ *   componentWillUnmount () {
+ *      this.keyboardDidShowListener.remove();
+ *      this.keyboardDidHideListener.remove();
+ *   }
+ *
+ *   _keyboardDidShow () {
+ *    window.alert('Keyboard Shown');
+ *   }
+ *
+ *   _keyboardDidHide () {
+ *    window.alert('Keyboard Hidden');
+ *   }
+ *
+ *   render() {
+ *     return (
+ *       <TextInput
+ *         onSubmitEditing={Keyboard.dismiss}
+ *       />
+ *     );
+ *   }
+ * }
+ *```
+ */
+module.exports = {
+
+  /**
+   * The `addListener` function connects a JavaScript function to an identified native
+   * keyboard notification event.
+   *
+   * This function then returns the reference to the listener.
+   *
+   * @param {string} nativeEvent The `nativeEvent` is the string that identifies the event you're listening for.  This
+   *can be any of the following:
+   * - `keyboardWillShow`
+   * - `keyboardDidShow`
+   * - `keyboardWillHide`
+   * - `keyboardDidHide`
+   * - `keyboardWillChangeFrame`
+   * - `keyboardDidChangeFrame`
+   *
+   * @param {function} jsFunction function to be called when the event fires.
+   */
+  addListener (nativeEvent: string, jsFunction: Function) {
+    return KeyboardEventEmitter.addListener(nativeEvent, jsFunction);
+  },
+
+  /**
+   * Removes all listeners for a specific event type.
+   *
+   * @param {string} eventType The native event string listeners are watching which will be removed.
+   */
+  removeAllListeners (eventType: string) {
+    KeyboardEventEmitter.removeAllListeners(eventType);
+  },
+
+  /**
+   * Removes a specific subscription.
+   *
+   * @param {EmitterSubscription} subscription The subscription emitter to be removed.
+   */
+  removeSubscription (subscription: Object) {
+    KeyboardEventEmitter.removeSubscription(subscription);
+  },
+
+  /**
+   * Dismisses the active keyboard and removes focus.
+   */
+  dismiss () {
+    dismissKeyboard();
+  }
+
+};

website/server/extractDocs.js

@@ -546,6 +546,7 @@ const apis = [
   '../Libraries/Image/ImageStore.js',
   '../Libraries/Components/Intent/IntentAndroid.android.js',
   '../Libraries/Interaction/InteractionManager.js',
+  '../Libraries/Components/Keyboard/Keyboard.js',
   '../Libraries/LayoutAnimation/LayoutAnimation.js',
   '../Libraries/Linking/Linking.js',
   '../Libraries/CustomComponents/ListView/ListViewDataSource.js',