This adds documentation about the O(1) scheduler to Documentation/.  The
new scheduler is complicated and providing future scheduler hackers some
background seems a Good Thing to me.

Specifically:

- add Documentation/sched-coding.txt: an overview of the functions,
  magic numbers, and variables in the scheduler as well as (most
  importantly) a review of the locking semantics.

- add Documentation/sched-design.txt: an edited version of Ingo's
  initial email to lkml about his scheduler.  Goes over the design,
  implementation, and goals of the scheduler.  I tried to edit it where
  needed to bring it in line with the scheduler as it is today.

- modify kernel/sched.c: update your copyright and add a change entry
  for the new scheduler.