[gnome-calendar] project: add example of commit message

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 713200773f980cf8aee2ba503a9a744b7b75615c
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Thu Sep 14 02:20:47 2017 -0300

    project: add example of commit message

 CONTRIBUTING.md |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 6bd22c1..65a9c3b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -9,7 +9,17 @@ Please note we have a code of conduct, please follow it in all your interactions
 
 1. Ensure your code compiles. Run `make` before creating the pull request.
 2. If you're adding new API, it must be properly documented.
-3. You may merge the pull request in once you have the sign-off of the maintainers, or if you
+3. The commit message is formatted as follows:
+   ```
+   component: <summary>
+
+   A paragraph explaining the problem and its context.
+
+   Another one explaining how you solved that.
+
+   <link to the bug ticket>
+   ```
+4. You may merge the pull request in once you have the sign-off of the maintainers, or if you
    do not have permission to do that, you may request the second reviewer to merge it for you.
 
 ## Code of Conduct