Add command in README

README.md

@@ -55,6 +55,7 @@ Prerequisites: Node.js 18+
 - `npm run preview` - Preview the production build
 - `npm run fetch-instances` - Fetch instances list from [instances.social](https://instances.social/), save it to `src/data/instances.json`
   - requires `.env.dev` file with `INSTANCES_SOCIAL_SECRET_TOKEN` variable set
+- `npm run sourcemap` - Run `source-map-explorer` on the production build
 
 ## Tech stack