Update contributing guidelines for AI-slop

**Describe what content should be added** :  

There is 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`


**Context** :  
Section: `contributing.md`

