add manifest description (#262)

assets/manifest.json

@@ -3,6 +3,7 @@
   "theme_color": "#4169e1",
   "name": "Pinafore for Mastodon",
   "short_name": "Pinafore",
+  "description": "Alternative web client for Mastodon, focused on speed and simplicity.",
   "display": "standalone",
   "start_url": "/?pwa=true",
   "icons": [