Add guidelines for AI contributions to the README.md

[avenmia]

As AI becomes more prominent in developer's workflows, we want to have a consistent policy to ensure code quality and prevent slop accumulation.

Some considerations for the guidelines:

• If the issue is marked "Good first issue", beginners should be encouraged to attempt it by hand first in order to understand the implementation
• Developers should be able to explain how the AI code works and how it fits into the bigger picture
• etc