Clarify how to add subcommands.

[bovender]

Describe the change

Updates the README.md file to help people understand that subcommands cannot be added to existing commands.

Why are we doing this?

Because auf #46, which also caused me some headaches.

Benefits

It helps users to better understand how teletype works.

Drawbacks

It does not address the problem the teletype crashes ungracefully when attempting to add a subcommand to an existing command.

Requirements

Put an X between brackets on each line if you have done the item:

• Tests written & passing locally?
• Code style checked?
• Rebased with master branch?
• Documentaion updated?

[piotrmurach]

Thank you! ❤️