Add Listmonk to mailing lists and newsletters.

README.md

@@ -350,6 +350,7 @@ _MDAs - IMAP/POP3 software_
 _Mailing lists servers and mass mailing software - one message to many recipients._
 
 - [Dada Mail](http://dadamailproject.com/) - Web-based list management system that can be used for announcement lists and/or discussion lists. ([Source Code](https://github.com/justingit/dada-mail)) `GPL-2.0` `Perl`
+- [Listmonk](https://listmonk.app/) - High performance, self-hosted newsletter and mailing list manager with a modern dashboard. ([Source Code](https://github.com/knadh/listmonk)) `AGPL-3.0` `Go`
 - [Mail For Good](https://github.com/freeCodeCamp/mail-for-good) `⚠` - Open source email campaign management tool for nonprofits. `BSD-3-Clause` `Javascript`
 - [Mailman](https://www.gnu.org/software/mailman/) - The Gnu mailing list server. `GPL-3.0` `Python`
 - [Mailtrain](https://mailtrain.org/) - self hosted newsletter application built on Node.js (v5+) and MySQL (v5.5+ or MariaDB). ([Source Code](https://github.com/andris9/mailtrain)) `GPL-3.0` `Nodejs`