docs: Add a screenshot to the README

Author: marien-probesys

README.md

@@ -1,7 +1,7 @@
 <h1 align="center">AgentJ</h1>
 
 <p align="center">
-    <strong>Libre antispam system with human authentication</strong><br>
+    <strong>Libre antispam system with human authentication.</strong><br>
     <a href="https://agentj.io/">agentj.io</a>
 </p>
 
@@ -16,6 +16,8 @@ Features include:
 - quota, shared mailbox, aliases;
 - and a lot more to come: [see our roadmap to learn more](/ROADMAP.md).
 
+![Screenshot of dashboard of AgentJ.](app/public/img/screenshot.webp)
+
 It runs on a Docker stack and uses well-known projects and technologies:
 
 - mail system: Postfix, Amavis, SpamAssassin, ClamAV, OpenDKIM