Documentation/vm/slub.txt: document slabinfo-gnuplot.sh

Add documentation on how to use slabinfo-gnuplot.sh script.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>