feat(getting-started): Add warning related to `direnv` installation

docs/op-stack/src/docs/build/getting-started.md

@@ -210,6 +210,8 @@ Once you’ve built both repositories, you’ll need head back to the Optimism M
     direnv allow .
     ```
 
+    If you need to install `direnv`, [make sure you also modify the shell configuration](https://direnv.net/docs/hook.html).
+
 1. Before we can create our configuration file, we’ll need to pick an L1 block to serve as the starting point for our Rollup. It’s best to use a finalized L1 block as our starting block. You can use the `cast` command provided by Foundry to grab all of the necessary information (replace `<RPC>` with the URL for your L1 Goerli node):
 
     ```bash