[Fedora-spins] spin-kickstarts branched for F15

[Bruno Wolff III]

Any changes destined for the F15 release should now be committed in the
F-15 branch of spin-kickstarts. Changes destined for F16 should be commited
to the master branch of spin-kickstarts.

Once final freeze starts (May 9th), changes to the F-15 branch should only
be made to fix NTH or blocker bugs.

The nightly composes should be switched over to using the the F-15 branch
until a bit after the F15 release.