add in a runtime example (#304)

* add in a runtime example

* updated based on feedback

runtime-examples/background.js

@@ -0,0 +1,17 @@
+let loadTime = new Date();
+let manifest = browser.runtime.getManifest();
+
+function onInstalledNotification(details) {
+  browser.notifications.create('onInstalled', {
+    title: `Runtime Examples version: ${manifest.version}`,
+    message: `onInstalled has been called, background page loaded at ${loadTime.getHours()}:${loadTime.getMinutes()}`,
+    type: 'basic'
+  });
+}
+
+function onClick() {
+  browser.runtime.reload();
+}
+
+browser.browserAction.onClicked.addListener(onClick);
+browser.runtime.onInstalled.addListener(onInstalledNotification);