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

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
 ```
 
@@ -617,6 +620,7 @@ More than one subsystem may be valid for any particular issue or pull request.
 [approved]: #getting-approvals-for-your-pull-request
 [benchmark results]: writing-and-running-benchmarks.md
 [collaborator guide]: collaborator-guide.md
+[git commit -s]: https://git-scm.com/docs/git-commit#Documentation/git-commit.txt--s
 [guide for writing tests in Node.js]: writing-tests.md
 [hiding-a-comment]: https://help.github.com/articles/managing-disruptive-comments/#hiding-a-comment
 [https://ci.nodejs.org/]: https://ci.nodejs.org/