CONTRIBUTING.md: add an extra example

This commit adds an extra example to submit an incrementing version
patch series to the mailing list which should be the continuation to
previous patch series.

This closes #1745

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

CONTRIBUTING.md

@@ -418,3 +418,10 @@ potentially interested reviewers.
 ```
 git send-email --to=git@vger.kernel.org --cc=<email1> --cc=<email2> [dir with patches]/*.patch
 ```
+
+To submit a nth version patch (say version 3):
+
+```
+git send-email --to=git@vger.kernel.org --cc=<email1> --cc=<email2> \
+    --in-reply-to=<the message id of cover letter of patch v2> [dir with patches]/*.patch
+```