Added acclaration about transitions in android

The navigation transitions doesn´t work in android <5.0 . it needs to be clarified

Author: elMuso

docs/core-concepts/navigation.md

@@ -496,7 +496,7 @@ frame.navigate(navigationEntry);
 
 ### Navigation transitions
 
-By default, all navigation will be animated and will use the default transition for the respective platform (UINavigationController transitions for iOS and Fragment transitions for Android). To change the transition type, set the `navigationTransition` property of the [`NavigationEntry`](http://docs.nativescript.org/api-reference/interfaces/_ui_frame_.navigationentry.html) to an object conforming to the [`NavigationTransition`](http://docs.nativescript.org/api-reference/interfaces/_ui_frame_.navigationtransition.html) interface.
+By default, all navigation will be animated and will use the default transition for the respective platform (UINavigationController transitions for iOS and Fragment transitions for Android). To change the transition type, set the `navigationTransition` property of the [`NavigationEntry`](http://docs.nativescript.org/api-reference/interfaces/_ui_frame_.navigationentry.html) to an object conforming to the [`NavigationTransition`](http://docs.nativescript.org/api-reference/interfaces/_ui_frame_.navigationtransition.html) interface. (For android this requires at least API 25 [Android 5.0])
 ``` JavaScript
 const getFrameById = require("tns-core-modules/ui/frame").getFrameById;