mod_cloud_notify_extensions: Add note about dependency
Noting here because people might not click through to see it on the
mod_cloud_notify_encrypted page.
Introduction
============
This module checks the IP addresses attempting to register an account
against a DNSBL, blocking the attempt if there is a hit.
Configuration
=============
Option Type Default
------------------- -------- ------------
registration\_rbl string *Required*
Compatibility
=============
Prosody Trunk
[1a0b76b07b7a](https://hg.prosody.im/trunk/rev/1a0b76b07b7a) or later.