Add Arabic (العربية) README translation

[wisdomqin]

Description

We currently have READMEs in English, Chinese, Japanese, Korean, and Spanish. Adding an Arabic translation would help reach more users.

What to do

1. Create README_ar.md based on the English README.md
2. Translate all content to Arabic
3. Add a link to the new file in the language selector at the top of all existing READMEs

Resources

• Existing READMEs for reference: README.md, README_zh-CN.md, etc.
• The language selector is the <p align="center"> block near the top of each file