v1.2.0: Update README.md

v1.0.0 - Initial Release

This is the initial release of the pytemperatureconverter Python package. It provides a simple and intuitive way to convert temperatures between four common scales:

• Celsius (°C)
• Fahrenheit (°F)
• Kelvin (K)
• Rankine (°R)

Features:

• Convert between Celsius, Fahrenheit, Kelvin, and Rankine.
• Includes warnings for physically impossible conversions (e.g., converting Kelvin to Celsius where values are out of range).
• Easy-to-use interface for both beginners and experts.