Indicate sw.js and background.js should be kept in sync

Change-Id: Ic8c3917f252a4a3817b510838ecac6ec37f0810c
diff --git a/src/background.js b/src/background.js
index 688aaa1..d7aae23 100644
--- a/src/background.js
+++ b/src/background.js
@@ -1,3 +1,5 @@
+// IMPORTANT: keep this file in sync with sw.js
+
 // When the extension gets updated, set new options to their default value.
 chrome.runtime.onInstalled.addListener(function(details) {
   if (details.reason == 'install' || details.reason == 'update') {