Adding instructions on using vapid key in window

Author: Matt Gaunt

README.md

@@ -56,6 +56,35 @@ a GCM API key and VAPID keys.
 
     webpush.sendNotification(pushSubscription, 'Your Push Payload Text');
 
+## Using VAPID Key for applicationServerKey
+
+When using your VAPID key in your web app, you'll need to convert the
+URL safe base64 string in a Uint8Array to pass into the subscribe call,
+which you can do like so:
+
+    function urlBase64ToUint8Array(base64String) {
+      const padding = '='.repeat((4 - base64String.length % 4) % 4);
+      const base64 = (base64String + padding)
+        .replace(/\-/g, '+')
+        .replace(/_/g, '/');
+
+      const rawData = window.atob(base64);
+      const outputArray = new Uint8Array(rawData.length);
+
+      for (let i = 0; i < rawData.length; ++i) {
+        outputArray[i] = rawData.charCodeAt(i);
+      }
+      return outputArray;
+    }
+
+    const vapidPublicKey = '<Your Public Key from generateVAPIDKeys()>';
+    const convertedVapidKey = urlBase64ToUint8Array(vapidPublicKey);
+
+    registration.pushManager.subscribe({
+      userVisibleOnly: true,
+      applicationServerKey: convertedVapidKey  
+    });
+
 # API Referance
 
 ## sendNotification(pushSubscription, payload, options)