[fractal] docs: Mention commit msg guidelines in CONTRIBUTING.md

[Julian Sparber <jsparber src gnome org>]

commit d1cbba5130bf1f13695e65c040ef85c8a86bef7e
Author: enterprisey <59171-enterprisey users noreply gitlab gnome org>
Date:   Wed Sep 29 18:49:33 2021 +0000

    docs: Mention commit msg guidelines in CONTRIBUTING.md

 CONTRIBUTING.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 97425020..507b6e98 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -16,3 +16,5 @@ To do this, just run `flatpak install --user org.freedesktop.Sdk.Extension.rust-
 We expect all code contributions to be correctly formatted.
 Before submitting a merge request, please run `cargo fmt` on your branch to ensure this is the case.
 It is also recommended to run `cargo clippy` as that will catch common errors and improve the quality of 
your submissions.
+
+Please follow the [GNOME commit message guidelines](https://wiki.gnome.org/Git/CommitMessages).