Add instructions to tighten security

Guides/nextcloud.md

@@ -52,3 +52,8 @@ $CONFIG = [
 
 No need to restart system, config is taken into account at web page reload.
 
+## Enforce security
+
+Follow [official instructions on Nextcloud doc](https://docs.nextcloud.com/server/latest/admin_manual/installation/harden_server.html)
+
+To enable *mod_headers* on HTTPD : `a2enmod headers`