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

Add -s option to examples using git commit
Add description for git commit -s

Signed-off-by: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>

Author: MikeMcC399

doc/contributing/pull-requests.md

@@ -151,7 +151,7 @@ to review changes that are split across multiple commits.
 
 ```bash
 git add my/changed/files
-git commit
+git commit -s
 ```
 
 Multiple commits often get squashed when they are landed. See the
@@ -206,6 +206,9 @@ A good commit message should describe what changed and why.
    This rule does not apply to dependency updates (e.g. cherry-picks), release
    commits, or backport commits.
 
+   [`git commit -s`][git commit -s] (with lowercase `s`) adds a
+   `Signed-off-by` trailer at the end of the commit log message.
+
 Sample final commit message after landing:
 
 ```text
@@ -312,7 +315,7 @@ GitHub will automatically update the pull request.
 
 ```bash
 git add my/changed/files
-git commit
+git commit -s
 git push origin my-branch
 ```
 
@@ -625,3 +628,4 @@ More than one subsystem may be valid for any particular issue or pull request.
 [nodejs/core-validate-commit]: https://github.com/nodejs/core-validate-commit/blob/main/lib/rules/subsystem.js
 [pull request template]: https://raw.githubusercontent.com/nodejs/node/HEAD/.github/PULL_REQUEST_TEMPLATE.md
 [running tests]: ../../BUILDING.md#running-tests
+[git commit -s]: https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--s