Add note about new assets and older react-native projects

Author: frantic

docs/Images.md

@@ -60,6 +60,8 @@ var icon = this.props.active ? require('./my-icon-active.png') : require('./my-i
 <Image source={icon} />
 ```
 
+NOTE: This system relies on build hooks for [Xcode](https://github.com/facebook/react-native/pull/3523) and [Gradle](https://github.com/facebook/react-native/commit/9dc036d2b99e6233297c55a3490bfc308e34e75f) that are included in new projects generated with `react-native init`. If you generated your projects before that, you'll have to manually add them to your projects to use the new images asset system.
+
 ## Images From Hybrid App's Resources
 
 If you are building a hybrid app (some UIs in React Native, some UIs in platform code) you can still use images that are already bundled into the app (via Xcode asset catalogs or Android drawable folder):