Add calculator script and README update (Session 4 exercise)

[mbecerra-apex]

This pull request completes the Session 4 Git practice exercise.

Changes included:

• Updated README.md with personal edit as required.
• Added a new folder (marymar_becerra) containing calculator.py.
• Implemented a simple Python calculator that:
  • Captures two numeric inputs (supports integers and decimals)
  • Supports +, -, *, and / operations
  • Validates user input
  • Handles division by zero

The script was developed and reviewed using GPT