CONTRIBUTING.md: Initial release
CONTRIBUTING.md: Initial release

As per issue #2623, it is important to define the commit guidelines.
This commit is about adding a first version of it.

Fixes: #2653
Signed-off-by: Erwan Velu <9949cc68615082c237e6af925cacbd2ea7f2e953@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible