Add comment in Example to know you must open module (#28)

* comment for noobs

this comment should help noobs in Reason that doesn't understand fully the module system yet.

* Update src/Example.re

Co-authored-by: Max Thirouin <git@moox.io>

Author: ViniciusAtaide

src/Example.re

@@ -1,3 +1,6 @@
+// Uncomment this to compile this example outside of this repo
+// in this example it's not necessary (since we are running it in the module repo itself)
+open ReactNavigation;
 module HomeScreen = {
   open ReactNative;
   [@react.component]