(docs) Add maintenance notice

@joshcooper is this the appropriate support tier?

Author: Ben Ford

README.md

@@ -5,6 +5,10 @@
 [![Modules Status](https://github.com/puppetlabs/puppetlabs-mailalias_core/workflows/Unit%20Tests%20with%20nightly%20Puppet%20gem/badge.svg?branch=main)](https://github.com/puppetlabs/puppetlabs-mailalias_core/actions) 
 [![Modules Status](https://github.com/puppetlabs/puppetlabs-mailalias_core/workflows/Unit%20Tests%20with%20released%20Puppet%20gem/badge.svg?branch=main)](https://github.com/puppetlabs/puppetlabs-mailalias_core/actions)
 
+> This module is maintained by Puppet, but we have no plans for future feature development. We will keep it working with current versions of Puppet, but new feature development will come from community contributions. It does not qualify for Puppet Support plans.
+>
+> [tier:maintenance-mode]
+
 
 #### Table of Contents