Add documentation for custom API key configuration

Discord Message

Author: philz42 Channel: #shelley Link: https://discord.com/channels/1405685085923049482/1450334528210993295/1455612114994729103

To plug in your own key, edit /etc/systemd/system/shelley.service and add a line like Environment=ANTHROPIC_API_KEY=xyz. Then edit /exe.dev/shelley.json and remove the "llm_gateway":"http://169.254.169.254/gateway/llm" section from the json. Finally, sudo systemctl daemon-reload and sudo systemctl restart shelley.