[NativeModules iOS] Remind people to rebuild

Was running into issues with this line, turned out I was forgetting to rebuild my project!

I'm sure there's better ways to word this, too. In general I think it would be nice to mention rebuilding somewhere near the top.

Author: raphaelrk

docs/native-modules-ios.md

@@ -62,7 +62,7 @@ RCT_EXPORT_METHOD(addEvent:(NSString *)name location:(NSString *)location)
 @end
 ```
 
-Now, from your JavaScript file you can call the method like this:
+Now, from your JavaScript file you can call the method like this (after making sure to rebuild):
 
 ```jsx
 import {NativeModules} from 'react-native';