docs: Update contributing guide with authentication setup instructions (

#261)

Co-authored-by: Eva Decker <[email protected]>

docs/CONTRIBUTING.md

@@ -41,6 +41,17 @@ Once your Convex account and project is created, a `.env.local` file will be gen
 > [!NOTE]
 > If this is your first time using Convex, [follow the tour](https://docs.convex.dev/get-started) to understand how the system works and read [The Zen of Convex](https://docs.convex.dev/zen) to understand best practices.
 
+### Set up authentication (first time only)
+
+Namesake authenticates users with [Convex Auth](https://labs.convex.dev/auth). To initialize authentication for local development, run:
+
+```shell
+npx @convex-dev/auth
+```
+
+> [!NOTE]
+> If you run into issues or prefer to set up authentication manually, refer to Convex Auth's [manual setup guide](https://labs.convex.dev/auth/setup/manual).
+
 ## Making changes
 
 ### Start the dev server