CONTRIBUTING: Add reference GNOME git guidelines
CONTRIBUTING: Add reference GNOME git guidelines

Add a reference to the GNOME git guidelines which all commits in this
git repository should strive to follow.

Signed-off-by: Viktor Sjölind <de9bdef9475b3a1b942c4473840195297b5e2350@pelagicore.com>

rpannek/pelux-manifests,DunderRoffe/pelux-manifests,Pelagicore/pelux-manifests,sashko/pelux-manifests