Update README to clarify getting started steps

The wording in the README leads you to believe that the `--create-principal` and `--fetch-role` will be created when running the docker command given. However, those two roles need to exist already in the user's account.

Start in README and locate the Docker command that mentions `--create-principal` and `--fetch-role`. Clarify that both roles must already exist in the user's account before running the command, then review the getting-started steps to ensure the wording no longer implies they are created automatically.

Written by the indexing model from the issue text.