OneSignal push notifications
The OneSignal push notification service is free and provides all the necessary features to notify your users of various activities as well as contact them via their dashboard. To enable it, you must first create an account with them. Once created and logged in, create a new app that will be used by your website to manage subscribers and send push notifications. When asked for a "platform type", select "web push" and hit "next".
On the next page, fill in your site's name and site URL.
You will need to create a new prompt for your app. What this does is basically displays either a small "subscribe" icon in the corner of your site or a notification slider at the top when your users first visit. You may choose whichever one you prefer.
Hit "save" at the bottom of the page once you're done with configuration. You will be asked to download "OneSignal SDK" files.
Download the package, unpack it and upload the files to your site in the same folder where your current software files are located, it should look like this:
Find your Application ID and API key from the OneSignal app settings page.
Paste them into the appropriate text boxes in the software control panel on the OneSignal configuration page and mark the OneSignal service as default. You should now see either a subscription notification prompt or a subscription icon when you visit your site.