doc: add contributing detail for git Signed-off-by trailer

[MikeMcC399]

Fixes: #64799

Situation

The instructions under contributing > Pull requests > Commit message guidelines document section are formulated without reference to the git commit -s option which provides for assisted addition of the required Signed-off-by trailer:

---

6. Your commit must contain the Signed-off-by line with your name and email
   address as an acknowledgement that you agree to the Developer Certificate of Origin.
   Bot generated commits are exempt from this requirement. If a commit has
   multiple authors, the Signed-off-by line should be added for each author;
   and at least one should match the author information in the commit metadata.
   This rule does not apply to dependency updates (e.g. cherry-picks), release
   commits, or backport commits.

---

The examples of git commit also do not show the use of the -s option.

It is however mentioned in the contributing > Guides and FAQs for first-time contributors > Going through the pull request review process

Change

Add instructions to contributing > Pull requests > Commit message guidelines for the git commit -s option.

Add the -s option to the existing git commit examples.

git commit -s, lower case s for Signed-off-by, could be confused with:

git commit -S, upper case S for --gpg-sign

so there is a note emphasising that lowercase s should be used.

[nodejs-github-bot]

Review requested:

• @nodejs/tsc

[bmuenzenmeyer]

LGTM - thanks for finding and fixing a real source of friction in the contributing process

[MikeMcC399]

LGTM - thanks for finding and fixing a real source of friction in the contributing process

I only really noticed there was a documentation gap when I tried mentoring a first-time contributor!

[nodejs-github-bot]

Landed in 0c87e08