sched: add CFS credits

add credits for recent major scheduler contributions:

  Con Kolivas, for pioneering the fair-scheduling approach
  Peter Williams, for smpnice
  Mike Galbraith, for interactivity tuning of CFS
  Srivatsa Vaddagiri, for group scheduling enhancements

Signed-off-by: Ingo Molnar <mingo@elte.hu>