[readme]: add note about GH CLI requirement

Author: legomushroom

README.md

@@ -1,9 +1,9 @@
 ## Codespaces Network Bridge
 
-<img width="749" alt="image" src="https://user-images.githubusercontent.com/1478800/161617508-b65de564-60f3-46c8-8394-5b28c8ac477b.png">
-
 🧪 *The extension is currently in Preview stage, so some hiccups are expected. Please help us to improve [by submitting feedback](https://github.com/github/gh-net#troubleshooting)!*
 
+<img width="749" alt="image" src="https://user-images.githubusercontent.com/1478800/161617508-b65de564-60f3-46c8-8394-5b28c8ac477b.png">
+
 This [GitHub CLI](https://cli.github.com/) extension allows to bridge network between a Codespace and your local machine, so the `Codespace` can reach out to any remote resource that is reachable from your machine. In another words, it uses your local machine as a network `gateway` to get to those resources.
 
 For instance, if you are using a `VPN` to connect to your enterprise network to access a database or any other remote resources on the private network, this extension allows you to get to those resources from within a Codespace, so that you can develop fully inside a Codespace!
@@ -24,6 +24,8 @@ gh extension install github/gh-net
     <img width="749" alt="image" src="https://user-images.githubusercontent.com/1478800/161620032-c999de5a-7445-4662-bedd-95be830556e9.png">
 </details>
 
+⚠️ If your Codespaces use non-default image, make sure that GitHub CLI is installed in the Codespaces. The installation step can happen either in `Dockerfile` or in one of the `.devcontainer.json` lifecycle hooks e.g. [postCreateCommand](https://code.visualstudio.com/docs/remote/devcontainerjson-reference). See https://github.com/cli/cli#installation for GitHub CLI installation instructions.
+
 <br />
 
 Picking up new extension version: