update sample image/readme (#148)

* update image and readme

* Update sample-app.png

* Update sample-app.png

Author: angelazhangmsft

samples/RNWinRTTestApp/README.md

@@ -7,6 +7,7 @@ The WinRT APIs demonstrated in this sample include:
 - `Windows.Foundation.Uri`
 - `Windows.Storage`
 - `Windows.Foundation.Diagnostics`
+- `Windows.UI.Notifications`
 
 ## Prerequisites