add Chinese (Simplified) README and language switcher entry

[MasterChiefCN]

What does this PR do?

• Adds a new README_ZH.md file with a complete Chinese (Simplified) translation of the project README.
• Adds a corresponding entry in the README language switcher (with China flag icon) so users can easily switch between English, Spanish, and Chinese.

Why?

To make the project more accessible to Chinese-speaking developers and lower the barrier to contribution.

Details

• The Chinese README covers [all sections / the core sections] of the original English README.
• Translation style: formal, aiming to be clear and consistent with the original.

Checklist

• README_ZH.md exists and is properly translated.
• The language switcher link points to the correct file.
• Flag icon displays correctly.
• All links in the Chinese README are verified.

[OpenSteam001]

thanks