perf kmem: Add help file

Add Documentation/perf-kmem.txt

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-mm@kvack.org <linux-mm@kvack.org>
LKML-Reference: <4B0B6EAF.80802@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/tools/perf/command-list.txt b/tools/perf/command-list.txt
index d3a6e18..02b09ea 100644
--- a/tools/perf/command-list.txt
+++ b/tools/perf/command-list.txt
@@ -14,3 +14,4 @@
 perf-top			mainporcelain common
 perf-trace			mainporcelain common
 perf-probe			mainporcelain common
+perf-kmem			mainporcelain common