document how to export consts from a swift native module

Summary:
I documented how to export consts from a swift native module.
Closes https://github.com/facebook/react-native/pull/15340

Differential Revision: D5556264

Pulled By: hramos

fbshipit-source-id: 0374f1c291e3c15424c8e53fef7ab30c60643165

docs/NativeModulesIOS.md

@@ -438,6 +438,10 @@ class CalendarManager: NSObject {
   func addEvent(name: String, location: String, date: NSNumber) -> Void {
     // Date is ready to use!
   }
+  
+  override func constantsToExport() -> [String: Any]! {
+    return ["someKey": "someValue"]
+  }
 
 }
 ```