Update README for CLI Deprecation

[danielhelfand]

🤔 What's the problem you've observed?

Number #478 outlines how the CLI will be deprecated. This caught me and others I work with off guard as we find the CLI really useful for our workflows. One thing to help prevent others from adopting the CLI as the primary method of building/running test moving forward would be to update the README to not mention the CLI as an option.

In the getting started workflow in the README, the CLI is the first method mentioned for running scenarios.

✨ Do you have a proposal for making it better?

Mention go test as the primary method for running scenarios drawing inspiration from here. It may also be wise to remove the CLI from the main README and migrate any of its docs to another document that makes it clear the tool is deprecated.

📚 Any additional context?

Again, it's unfortunate to see the CLI deprecated, so it might be helpful to create a small migration guide for those who were using it. Happy to contribute to this issue and add a guide based on our own experience migrating to go test if that would be helpful for others.