clarify a message in preferences.xml

Summary:
Especially for newcomers, this may help since some may think the effect is immediate.
Closes react#12066

Differential Revision: D4500929

Pulled By: mkonicek

fbshipit-source-id: dfa05134f208c084dacb3e490fe9eb8df323ffd5

Author: vonovak

ReactAndroid/src/main/res/devsupport/xml/preferences.xml

@@ -10,7 +10,7 @@
     <CheckBoxPreference
         android:key="js_dev_mode_debug"
         android:title="JS Dev Mode"
-        android:summary="Load JavaScript bundle with __DEV__ = true for easier debugging.  Disable for performance testing."
+        android:summary="Load JavaScript bundle with __DEV__ = true for easier debugging.  Disable for performance testing. Reload for the change to take effect."
         android:defaultValue="true"
         />
     <CheckBoxPreference