[PATCH cgroup/for-3.16] cgroup: add documentation about unified hierarchy

[Tejun Heo]

Hello,

Unified hierarchy is finally out for review [1][2].  This patch adds
the documentation which describes the design and rationales.  If you
can think of more people to cc, please go ahead.

If you have any comments and/or questions, please don't hesitate.

Thanks.

[1] http://lkml.kernel.org/g/1397511430-2673-1-git-send-email-tj@xxxxxxxxxx
[2] http://lkml.kernel.org/g/1397511846-2904-1-git-send-email-tj@xxxxxxxxxx

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