[PATCH] CPUSETS: add mems to basic usage documentation

It seems that there must be at least one node in mems and at least one CPU
in cpus in order to be able to assign tasks to a cpuset.  This makes sense.
 And I think it would also make sense to include a mems setting in the
basic usage section of the documentation.

I also wonder if something logged to dmsg, explaining why a write failed,
would be a good enhancement.  I ended up having rummage arround in cpuset.c
in order to work out why my configuration was failing.

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Paul Jackson <pj@sgi.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>