Update fedora_bz.py usage for recent changes, add it for Changes
    
    I made some significant changes to the fedora_bz.py script today.
    This updates the docs to reflect current usage of the script
    after these changes. It also changes the changes-contingency
    page to recommend using the script - currently the page does not
    say to use the script even though it has the capability to do
    the necessary work for the change deadlines.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>