Docs: Adding citrus integration testing framework as reference

Author: marcelmay

docs/index.html

@@ -318,6 +318,8 @@ <h5>Projects extending/wrapping GreenMail:</h5>
                   <li><a href="https://github.com/greenmail-mail-test/greenmail-client-integrations">
                      Example container integrations of GreenMail and various web based mail clients
                     </a></li>
+                  <li><a href="https://citrusframework.org/">Citrus Integration
+Testing</a> used GreenMail for mocking mail server</li>
                   <li><a href="https://spring.io/blog/2021/05/19/spring-integration-5-5-goes-ga">Spring Integration 5.5 uses now GreenMail for email integration testing</a>
                     </li>
                     <li><a href="https://github.com/davidnewcomb/greenmail-http">GreenMail HTTP</a>