Clears up what the TUI is and how to start it.

[johnnymatthews]

Proposed Changes

• Updates the output of the CLI when using the --help argument:

Usage: entropy [options] [command]

CLI interface for interacting with entropy.xyz. Running this binary without any commands or arguments starts a text-based interface.

Options:
  -e, --endpoint <endpoint> 

Checklist

• I have performed a self-review of my code.
• I have added tests.
• I have commented my code.
• I have included a CHANGELOG.md entry.
• I have updated documentation in github.com:entropyxyz/entropy-docs, where necessary.

Closes #180