Fix review comments: https://github.com/mdn/webextensions-examples/issues/30#issuecomment-172649839

* added README
* moved listeners above addListener calls

notify-link-clicks-i18n/background-script.js

@@ -1,8 +1,3 @@
-/*
-Assign `notify()` as a listener to messages from the content script.
-*/
-chrome.runtime.onMessage.addListener(notify);
-
 /*
 Log that we received the message.
 Then display a notification. The notification contains the URL,
@@ -19,3 +14,8 @@ function notify(message) {
     "message": content
   });
 }
+
+/*
+Assign `notify()` as a listener to messages from the content script.
+*/
+chrome.runtime.onMessage.addListener(notify);