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

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',