docs: Add section on handling missing arguments

[markrahimi]

Summary

This PR addresses issue #633 by adding a new documentation section on handling missing arguments in Python Fire CLIs.

Changes

• Added a new "Handling Missing Arguments" section to the README
• Included a practical example showing how to use default values and docstrings
• Explained how this improves user experience when required arguments are missing

Benefits

• Users will get clearer error messages through better documentation
• Provides best practices for handling required vs optional arguments
• Makes the --help output more informative

Closes #633

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