docs: add EmailEngine integration note to README and CLI docs

Add note highlighting that mailauth is used by EmailEngine for
validating email authentication settings, with link to the
Email Authentication Testing documentation.

Author: andris9

README.md

@@ -17,6 +17,9 @@
 
 mailauth is a pure JavaScript implementation, requiring no external applications or compilation. It runs on any server or device with Node.js version 16 or later.
 
+> [!NOTE]
+> mailauth is used by [EmailEngine](https://emailengine.app/) for validating email authentication settings. See the [Email Authentication Testing documentation](https://learn.emailengine.app/docs/advanced/email-authentication-testing) for details.
+
 ## Table of Contents
 
 1. [Installation](#installation)

cli.md

@@ -4,6 +4,9 @@
 
 mailauth provides a command-line utility for email authentication, complementing its [Node.js library](README.md). This guide explains how to use the mailauth CLI to perform various email authentication tasks.
 
+> [!NOTE]
+> mailauth is used by [EmailEngine](https://emailengine.app/) for validating email authentication settings. See the [Email Authentication Testing documentation](https://learn.emailengine.app/docs/advanced/email-authentication-testing) for details.
+
 ## Table of Contents
 
 - [Installation](#installation)