Add a statement on usage of AI tools

[eduardo-rodrigues]

It seems like a good idea to introduce a statement on the usage of AI tools based on recent experience from recent community contributions and on discussions held within the GSoC programme mailing list. Maybe a simple pull request template will do the job? Or a subsection on the README?

Comments and ideas welcome.

Ping @wdconinc, @maszyman, @vvolkl, @aaronj0 for input from our GSoC coordinators.

[wdconinc]

How about an organization-wide pull request template (https://github.com/HSF/.github), as .github/pull_request_template.md with a simple checkbox?

<!-- Describe your pull request at the top, and take note of the checkbox below -->



### Please check if any of the following apply
- [ ] AI was used in preparing this PR. Please describe usage below.

[vvolkl]

In principle, I like what the linux kernel now has: https://github.com/torvalds/linux/blob/master/Documentation/process/coding-assistants.rst

But that should probably go to CONTRIBUTING.md