Add a contributing file that limits AI use

[diondokter]

Copy from https://github.com/diondokter/device-driver/blob/master/CONTRIBUTING.md

This codebase is very subtle and even the tests and fuzzer can miss things.
So I don't trust AI to do a good enough job here, in addition to the other reasons not to use AI.

The only PR I recall that might not meet the criteria from the new AI guidelines is: #123
But that's only a suspicion. I'll leave that code in since it doesn't touch any core component.

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
 18704	     56	   1728	  20488	   5008	example

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