Add AI usage policy to CONTRIBUTING.md

[goneall]

Added AI usage policy to guide contributors on the use of AI tools.

[pmonks]

A very timely addition! I'm sure you've looked at prior art in crafting this, but FWIW I quite like the OpenJDK interim policy on generative AI - I think language like theirs makes it even clearer about what these tools can (and more importantly cannot) be used for:

Contributions in the OpenJDK Community must not include content generated, in part or in full, by large language models, diffusion models, or similar deep-learning systems. Content, in this context, includes but is not limited to source code, text, and images in OpenJDK Git repositories, GitHub pull requests, e-mail messages, wiki pages, and JBS issues.

Contributors in the OpenJDK Community may use generative AI tools privately to help comprehend, debug, and review OpenJDK code and other content, and to do research related to OpenJDK Projects, so long as they do not contribute content generated by such tools.

[goneall]

Credit to @bact for pointing me to the AI policy used for the NTIA Conformance Checker (you'll notice a striking similarity ;).

Probably worth more discussion on how strict we want to be and also if we want to have a general policy for all SPDX containing code.

I'm going to convert this to draft for now.