Add helloworld app to catalog. Helloworld_ynh is in v2 packaging, this is meant to be a demo of packaging *and* test the inclusion of v2 apps in the catalog

apps.json

@@ -1420,6 +1420,11 @@
         ],
         "url": "https://github.com/YunoHost-Apps/hedgedoc_ynh"
     },
+    "helloworld": {
+        "category": "dev",
+        "state": "working",
+        "url": "https://github.com/YunoHost-Apps/helloworld_ynh"
+    },
     "hextris": {
         "category": "games",
         "high_quality": true,