Replace link to Gitter with Discord in README.md
:books: <a href="https://doc.crowdsec.net">Documentation</a>
:diamond_shape_with_a_dot_inside: <a href="https://hub.crowdsec.net">Configuration Hub</a>
:speech_balloon: <a href="https://discourse.crowdsec.net">Discourse (Forum)</a>
-:speech_balloon: <a href="https://gitter.im/crowdsec-project/community?utm_source=share-link&utm_medium=link&utm_campaign=share-link">Gitter (Live chat)</a>
+:speech_balloon: <a href="https://discord.gg/wGN7ShmEE8">Discord (Live Chat)</a>
</p>
