[enh] add experimental warning for config-panel

src/locales/en.json

@@ -136,6 +136,7 @@
     "error_server": "Server error",
     "error_server_unexpected": "Unexpected server error (%s)",
     "everyone_has_access": "Everyone has access.",
+    "experimental_warning": "Warning: this feature is experimental and not consider stable, you shouldn't be using it except if you know what you are doing.",
     "filesystem": "Filesystem",
     "firewall": "Firewall",
     "footer_version" : "Powered by <a href='https://yunohost.org'>YunoHost</a> %s (%s).",

src/views/app/app_config-panel.ms

@@ -7,6 +7,9 @@
 
 <div class="separator"></div>
 
+<div class="alert alert-warning" role="alert">{{t 'experimental_warning'}}</div>
+
+
 {{#if config_panel}}
 
 <div class="panel-group" id="accordion" role="tablist" aria-multiselectable="true">