Add more fields to PWA manifest (#4145)

assets/site.webmanifest

@@ -15,5 +15,7 @@
     ],
     "theme_color": "#575757",
     "background_color": "#575757",
-    "display": "standalone"
+    "display": "standalone",
+    "description": "An alternative front-end to YouTube",
+    "start_url": "/"
 }