Contribution guidelines update regarding tool usage

[Florian Weimer]

I would like to make the following changes to “5. Commit formatting” in
<https://sourceware.org/glibc/wiki/Contribution%20checklist>.

Under “5.2 Commit message body”, add bullet point:

* Mention any tools used to auto-generate source code that is added with
  the change.

Under ‘5.3 Attribution”, add bullet point:

* Reviewers may request that you add an “Assisted-by” Git trailer if
  parts of the contribution have been auto-generated.


I believe this matches existing practice when it comes to generating
files with (say) autoconf and MPFR.  It also covers the special case
when contributors want to check in compiler output and want to maintain
the output going forward as source code (this has come up before in the
context of libmvec).  It partly addresses recent discussions regarding
LLM usage and disclosure.

I believe this still meets the FSF request to wait until they have an
LLM policy for the GNU project.

Thoughts?  Concerns?

Thanks,
Florian