fixed readme (#26)

README.md

@@ -115,6 +115,12 @@ You will see all the logs of the system running normally, including the successf
 ## 🧪 Experiments
 ### Agent Baselines
 If you wish to run the baseline agent used in our paper, you can execute the following command as an example under the GPT-4V pure-screenshot setting:
+
+Set **OPENAI_API_KEY** environment variable with your API key
+```bash
+export OPENAI_API_KEY='changme'
+```
+
 ```bash
 python run.py --path_to_vm Ubuntu/Ubuntu.vmx --headless --observation_type screenshot --model gpt-4-vision-preview --result_dir ./results
 ```