docs(push): restore push action buttons README section (#444)

This reverts commit 5e82562.

Re-applying README documentation for push action buttons ahead of the 4.4.0 release.

Author: evan-masseau

README.md

@@ -28,6 +28,7 @@ send them timely push notifications via [FCM (Firebase Cloud Messaging)](https:/
   - [Collecting Push Tokens](#collecting-push-tokens)
   - [Receiving Push Notifications](#receiving-push-notifications)
     - [Rich Push](#rich-push)
+    - [Push Action Buttons](#push-action-buttons)
     - [Tracking Open Events](#tracking-open-events)
     - [Silent Push Notifications](#silent-push-notifications)
     - [Custom Data](#custom-data)
@@ -479,6 +480,11 @@ attaching it to the notification is handled within `KlaviyoPushService`. If an i
 (e.g. if the device has a poor network connection) the notification will be displayed without an image
 after the download times out.
 
+#### Push Action Buttons
+[Push Action Buttons](https://help.klaviyo.com/hc/en-us/article/46285872166683) provide the ability to add clickable buttons to
+push notification messages. These buttons can show custom text, and, when clicked, deep link or open your app.
+A notification can include up to 3 buttons. No additional SDK setup is required.
+
 #### Tracking Open Events
 To track push notification opens, you must call `Klaviyo.handlePush(intent)` when your app is launched from an intent.
 This method will check if the app was opened from a notification originating from Klaviyo and if so, create an