Update contributing guidelines for AI-slop

[jgadsden]

Describe what content should be added :

There are an increasing amount of submissions that are using generative AI;
it would be good to provide clear guidance on what is acceptable and what is not, similar to :
https://github.com/OWASP/threat-dragon/blob/main/contributing.md#use-of-ai

Declaration:

Please make sure you have:

• read the contribution guide and agree to the Code of Conduct
• not used agentic or generative AI in creating this issue

Context :
Section: contributing.md