Add how to create VAPID_PRIVATE_KEY and VAPID_PUBLIC_KEY (#376)

Running-Mastodon/Docker-Guide.md

@@ -41,6 +41,10 @@ Now the image can be used to generate secrets. Run the command below for each of
 
     docker-compose run --rm web rake secret
 
+To enable Web Push notifications, you should generate a few extra secrets and put them into your `.env.production` file. Run Command below below for each of `VAPID_PRIVATE_KEY` and `VAPID_PUBLIC_KEY` then copy the result into the `.env.production` file: 
+
+    docker-compose run --rm web rake mastodon:webpush:generate_vapid_key
+
 Then you should run the `db:migrate` command to create the database, or migrate it from an older release:
 
     docker-compose run --rm web rake db:migrate