Added Citadel mail server

Added Citadel mail server in "complete solution mail servers" category

README.md

@@ -325,6 +325,7 @@
 
 * [Mail-in-a-Box](https://mailinabox.email/) - Take back control of your email with this easy-to-deploy mail server in a box.
 * [iRedMail](http://www.iredmail.org/) - Full-featured mail server solution based on Postfix and Dovecot.
+* [Citadel](http://www.citadel.org/) - Feature packed, easy, versatile, and powerful mail server, thanks to exclusive "rooms" based architecture.
 
 
 ## Monitoring