Mention Sprockets config in deploy template

lib/kamal/cli/templates/deploy.yml

@@ -77,6 +77,10 @@ registry:
 # Bridge fingerprinted assets, like JS and CSS, between versions to avoid
 # hitting 404 on in-flight requests. Combines all files from new and old
 # version inside the asset_path.
+#
+# If your app is using Sprockets gem, ensure it has `config.assets.manifest`
+# set. See https://github.com/basecamp/kamal/issues/626 for details
+#
 # asset_path: /rails/public/assets
 
 # Configure rolling deploys by setting a wait time between batches of restarts.