add in a runtime example (#304)

* add in a runtime example

* updated based on feedback

runtime-examples/README.md

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+## What it does
+
+Trigger an install notification when you install it. On the browser action text is
+shown showing when the background page was last loaded in minutes and seconds.
+
+Clicking the browser action triggers a reload and updates the background page load time.
+
+The notifications show the version of the extension by fetching the manifest.
+
+## What it shows
+
+Demonstration of some runtime and notification APIs.
+
+Icon is from: https://www.iconfinder.com/icons/172151/run_icon#size=128

runtime-examples/background.js

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+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);

runtime-examples/manifest.json

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+{
+  "browser_action": {
+    "default_icon": "run.png"
+  },
+  "description": "Demonstration of some runtime APIs",
+  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/runtime-examples",
+  "manifest_version": 2,
+  "name": "Runtime examples",
+  "version": "1.0",
+  "permissions": ["notifications"],
+  "background": {
+    "scripts": ["background.js"]
+  },
+  "icons": {
+    "128": "run.png"
+  }
+}