added short comments for all JS functions

notify-link-clicks/content-script.js

@@ -1,5 +1,12 @@
+/*
+Add notifyExtension() as a listener to click events.
+*/
 window.addEventListener("click", notifyExtension);
 
+/*
+If the click was on a link, send a message to the background page.
+The message contains the link's URL.
+*/
 function notifyExtension(e) {
   var target = e.target;
   while ((target.tagName != "A" || !target.href) && target.parentNode) {