Doc: add a design template
    
    Add a template for new features in doc/designs/template.md
    The template is excluded from automatic doc generation.
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>