Update README.md

Updates README.md to clarify the behavior of Auth vs. OPENAI_API_KEY

README.md

@@ -116,7 +116,9 @@ If you prefer to pay-as-you-go, you can still authenticate with your OpenAI API
 export OPENAI_API_KEY="your-api-key-here"
 ```
 
-> Note: This command only sets the key for your current terminal session, which we recommend. To set it for all future sessions, you can also add the `export` line to your shell's configuration file (e.g., `~/.zshrc`).
+> Notes:
+> This command only sets the key for your current terminal session, which we recommend. To set it for all future sessions, you can also add the `export` line to your shell's configuration file (e.g., `~/.zshrc`).
+> If you have signed in with ChatGPT, Codex will default to using your ChatGPT credits. If you wish to use your API key, use the `/logout` in the tui to clear your auth data.
 
 ### Choosing Codex's level of autonomy