F21 milestone kernel versions

[Josh Boyer]

Just so there's one place to point people:

F21 Alpha: 3.16.y (likely 3.16.1 at this point, but depends on further slips)

F21 Beta/RC/GA:  Unknown.  We'll revisit it once Alpha is out the door
(because it depends on how many more times Alpha is going to slip,
etc).

josh