Update Keyboard.js

Summary:
What existing problem does the pull request solve?

According to this issue: https://github.com/facebook/react-native/issues/3468 , only `keyboardDidShow`and `keyboardDidHide` events are available on android at this moment.

I think this information should be displayed in the documentation since default `android:windowSoftInputMode` is `adjustResize`.
Closes https://github.com/facebook/react-native/pull/13155

Differential Revision: D4795828

Pulled By: hramos

fbshipit-source-id: 2c114f3040808a5cc3cdeb29b2067877df353620

Libraries/Components/Keyboard/Keyboard.js

@@ -99,6 +99,9 @@ let Keyboard = {
    * - `keyboardWillChangeFrame`
    * - `keyboardDidChangeFrame`
    *
+   * Note that if you set `android:windowSoftInputMode` to `adjustResize`  or `adjustNothing`,
+   * only `keyboardDidShow` and `keyboardDidHide` events will available on Android.
+   *
    * @param {function} callback function to be called when the event fires.
    */
   addListener(eventName: KeyboardEventName, callback: KeyboardEventListener) {