Create offline package if not running in "Debug" config

Summary:**Problem**: As seen in facebook/react-native#5820, many devs are confused by the fact that the offline bundle is not generated when running against the simulator, even when running in the "Release" configuration which is supposed to mimic "production" scenarios.

This pull request is a small change that fixes facebook/react-native#5820 by updating the `react-native-xcode.sh` shell script to still generate the ofline bundle during Release configuration. It also updates `AppDelegate.m` to better document this behaviour in the comments so as to avoid any surprises.

**Test plan**: This is a simple change, the two tests done were
1. In a new React Native project, verify that an offline build is not generated when running against the simulator in "Debug" configuration as per normal.
2. Change to a "Release" configuration via Product > Scheme > Edit Scheme in XCode, then verify that the project runs with the offline build generated.

![screen shot 2016-02
Closes facebook/react-native#6119

Differential Revision: D2970755

Pulled By: javache

fb-gh-sync-id: 64f658512869c73aa19286ca1e3dc6e31b5ac617
shipit-source-id: 64f658512869c73aa19286ca1e3dc6e31b5ac617

Author: geof90

generator-ios/templates/app/AppDelegate.m

@@ -36,7 +36,9 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   /**
    * OPTION 2
    * Load from pre-bundled file on disk. The static bundle is automatically
-   * generated by "Bundle React Native code and images" build step.
+   * generated by the "Bundle React Native code and images" build step when
+   * running the project on an actual device or running the project on the
+   * simulator in the "Release" build configuration.
    */
 
 //   jsCodeLocation = [[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];