Documentation: contributing: Add commit message guidelines

All developers, whether junior or experienced, can benefit from
improving their commit message writing skills. Add a paragraph to the
contribution documentation to explain this, with a link to a good guide.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

Documentation/contributing.rst

@@ -68,6 +68,13 @@ code that is as easy to read, understand and maintain as possible. This is
 made possible by a set of :ref:`coding-style-guidelines` that all submissions
 are expected to follow.
 
+We also care about the quality of commit messages. A good commit message not
+only describes what a commit does, but why it does so. By conveying clear
+information about the purpose of the commit, it helps speeding up reviews.
+Regardless of whether you're new to git or have years of experience,
+https://cbea.ms/git-commit/ is always a good guide to read to improve your
+commit message writing skills.
+
 The patch submission process for libcamera is similar to the Linux kernel, and
 goes through the `libcamera-devel`_ mailing list. If you have no previous
 experience with ``git-send-email``, or just experience trouble configuring it