Document how to use index.js as entry point (#2412)

* Document how to use index.js as entry point (ios)

* android too

* android too (fixed)

docs/installation-android.md

 	}
 	```
 
+	Also, add the following
+
+   ```java
+   @Override
+   public String getJSMainModuleName() {
+   	return "index";
+   }
+   ```
+
+   if you are using `index.js` as your entry point instead of `index.ios.js` and `index.android.js` (it is the default since React Native 0.49).
+
 	Make sure that `isDebug` and `createAdditionalReactPackages` methods are implemented.
 
 6. Update `AndroidManifest.xml` and set **android:name** value to `.MainApplication`
 		...
 	/>
 	```
-

docs/installation-ios.md

     ```
 
 Its content must be replaced with the content of this [reference](https://github.com/wix/react-native-navigation/blob/master/example/ios/example/AppDelegate.m)
+
+Replace `@"index.ios"` with `@"index"` if you are using `index.js` as your entry point instead of `index.ios.js` and `index.android.js` (it is the default since React Native 0.49).