Contribution guidelines update regarding tool usage

[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". 
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".