Contribution guidelines update regarding tool usage

[Florian Weimer]

* Paul Eggert:

> Thanks, those suggestions make sense. One little improvement:
>
> On 2026-04-17 05:52, Florian Weimer wrote:
>
>> * Reviewers may request that you add an “Assisted-by” Git trailer if
>>    parts of the contribution have been auto-generated.
>
> It would be helpful to give an example, as we can't assume that every
> new contributor is up to speed on what to put into
> "Assisted-by".

It's more of a guideline for the reviewers for sure.  New contributors
are unlikely to read *that* far in the contribution guidelines.

> Something like this, perhaps:
>
> * Reviewers may request that you add an “Assisted-by” line in your
>   commit message if parts of the contribution have been
>   auto-generated. For example, "Assisted-by: Claude Opus 4.7 and
>  Autoconf 2.73".

Is there a sufficiently open LLM we could mention instead?  Otherwise I
would go with just mentioning Autoconf.

Thanks,
Florian