Explain purpose of timer thread.

extension/background/timerThread.js

@@ -1,3 +1,10 @@
+/**
+ * This file should be used as a WebWorker.
+ * Background pages in the WebExtensions model do
+ * not allow to schedule callbacks that should be called
+ * after a timeout.  We can emulate this with WebWorkers.
+ */
+
 onmessage = function(e) {
   self.setInterval(() => postMessage(true), e.data.interval);
 }