Merge pull request #1336 from mlescaudron/patch-1

Remove duplicate link to Citadel (Mail servers + Groupware)

README.md

@@ -286,7 +286,6 @@ _CalDAV or CardDAV clients._
 
 _Simple deployment of a mail server, e.g. for inexperienced or impatient admins._
 
-- [Citadel](http://www.citadel.org/) - Feature packed, easy, versatile, and powerful mail server, thanks to exclusive "rooms" based architecture. ([Source Code](http://code.citadel.org/?p=citadel.git;a=summary)) `GPL-3.0` `C`
 - [docker-mailserver](https://github.com/tomav/docker-mailserver) - Fullstack but simple mail server (smtp, imap, antispam, antivirus, etc.). Only configuration files, no SQL database. Keep it simple and versioned. Easy to deploy and upgrade. `MIT` `Docker`
 - [Inboxen](https://inboxen.org) - Inboxen is a service that provides you with an infinite number of unique inboxes. ([Source Code](https://github.com/Inboxen/Inboxen)) `GPL-3.0` `Python`
 - [iRedMail](http://www.iredmail.org/) - Full-featured mail server solution based on Postfix and Dovecot. ([Source Code](https://bitbucket.org/zhb/iredmail/commits/)) `GPL-3.0` `Shell`