Add docs about required lotus flags/settings in byo node

[andrewxhill]

notes:

- LOTUS_API_LISTENADDRESS=/ip4/0.0.0.0/tcp/1234/http (you can avoid this if don't need to listen in localhost)
      - LOTUS_CLIENT_USEIPFS=true (set this)
      - LOTUS_CLIENT_IPFSUSEFORRETRIEVAL=true (set this)
      - LOTUS_CLIENT_IPFSMADDR=/dns4/ipfs/tcp/5001 (you should point this to the Powergate IPFS node: /ip4/<IP of the IPFS node>/tcp/5001 )
The docker-compose file has all the necessary wiring there. If you replicate the env configs, you should be good to go.